A Team's Roles
While most teams vary in exact heroes used, every MOBA team uses a combination of these basic roles.
Jungler
A jungler is a character who clears out enemies inside the jungle beneath the lanes to gain unique combat buffs and gain XP quickly for the whole team.
Passive Jungler- A passive jungler is a jungler who pays no attention to the lanes. Passive junglers are usually more focused on getting XP buffs and the prime orb.
Ganker- A ganker is a jungler who, in addition to jungling, picks off enemies by using high burst damage abilities before returning to the shadow of the jungle.
Laner
Laners are the body of your team. A laner is a character who stays on the lanes, either destroying enemy towers or playing defensively. There are many types of laners that can be used.
Carry- A carry is the most basic form of laner. Carries deal high single target damage that can help them take out towers and heroes quickly.
Tank- Tanks are characters with high health that protect the other lanes.
Support- Support characters protect their allies from afar with shielding or healing abilities.
CC- CC (crowd control) characters are characters that use effects that slow, stun or otherwise control the location and effectiveness of enemies.
AoE- An AoE character is a character who uses abilities that deal damage to multiple things at once. AoE characters are best used alongside CC characters, as CC allows the AoE to hit the most amount of things by controlling where things are.
Heroes
Every hero has their own role that they are attuned to:
Dekker- CC
Dekker has the best crowd control in the game, but the lack of any DpS abilities makes her weak without an AoE character backing her up.
Feng Mao- Carry/Passive Jungler/Ganker
Feng Mao is a strong jungler because he is a stack character, meaning that he gains damage buffs over time to help him in the late game. He is also a decent laner, but he isn't very effective in the early game.
Gadget- Carry/CC
Gadget has a strong DpS arsenal and good slow effects, making her ideal in many situations.
Gideon- Carry/AoE
Gideon is unique in that his character has multiple strong AoE effects, making him almost necessary coupled with any CC characters.
Grux- Carry/Passive Jungler/Ganker
Grux is a strong jungler because he is a stack character, but his stacks only gain him 200 health for 100 kills, making him a relatively weak jungler. However, he has strong DpS abilities, making him decent at carrying.
Howitzer- Carry
Howitzer is a fairly generic character with a good DpS arsenal.
Kallari- Ganker
Kallari is probably the best ganker and maybe even the best jungler, as he deals lots of burst damage and has good escape abilities.
Murdock- Carry
Murdock is probably the best carry in the game right now, as his ultimate is ridiculously powerful. I recommend jamming Murdock into most team compositions.
Muriel- Support
Muriel is a very strong support character, and is also the only support character. At the moment, Muriel is probably the best character in the game and should always be used unless you have a specific reason not to.
Rampage- Tank/Passive Jungler
Rampage is a good jungler, as his ridiculous health regen in the jungle lets him live indefinitely, gaining XP buffs faster than other heroes. He is also a strong tank, as his health regen is still effective in lanes and gives Rampage good survivability.
Sevarog- Passive Jungler/Ganker
Sevarog is a stack character who gains both health and damage with kills, making him a good jungler. He also has ganking potential as his E ability slows fleeing enemies and his ultimate can knock them back into the battle.
Sparrow- Carry
Sparrow is just a strong DpS character.
Steel- Tank/CC/Carry
Steel has high health and a shielding ability, making him ideal for tanking. He also has two effective CC abilities. Thirdly, he can be built to both soak up and deal damage, which is good for a carry.
Twinblast- Carry/Ganker
Twinblast is one of the stronger carries, as his DpS arsenal is slightly better than that of other characters. He can also gank decently.
Composition of Roles
There are many viable role compositions. Here are some outlines of role composition:
5 Laners
This strategy is high risk, high reward. While you have lots of early game flexibility and have big advantages, failing in the early game means you will slowly be pushed back until you lose. One possible alteration of this strategy would be to rush 5 at the beginning, then if that fails, have a jungler drop back and settle into 4 laners, 1 jungler.
4 Laners, 1 Jungler
This is probably the most common strategy used. With four laners, you still have the role flexibility of five laners with the addition of a jungler to help your late game. This strategy is very balanced and requires less skill and teamwork than other combinations.
3 Laners, 2 Junglers
This strategy requires both junglers to be able to work together as the rotate around the jungle, killing jungle creatures and ganking enemy heroes. This strategy relies on you to be able to win the late game, as you don't have enough laners to make solid pushes in the early to mid game. The junglers need to be very familiar with each other and also need to be on comms together.
Fitting Heroes into Compositions
This is probably the hardest task to do when building a team, because you have to keep cross-hero synergy in mind at all times.
5 Laners
With 5 laners, want to push hard early to remove multiple inhibitors quickly. This probably means a 3-0-2 setup, where you have one squad pushing left and the other pushing right, with both able to fall back on mid if necessary. For each, you'll most likely want a tank and a carry. I would combine Steel and Gideon on one side and Rampage and any carry on the other side. Then you'll probably want to use Muriel as a fifth laner on either side. Murdock truly isn't effective enough to be useful in this setup because his ultimate is his best character feature and if you do this correctly, you won't use the ultimate enough to make Murdock worth it.
4 Laners, 1 Jungler
This strategy is probably the most flexible composition, since you can spread your players out in various ways. I would generally spread out either 1-2-1 or 2-0-2. A 1-2-1 setup is the most balanced setup. It generally uses one carry in each lane, with a support character moving between lanes. This means that a setup with Murdock, two other carries and Muriel laning might be strongest. With a 1-2-1 setup, a ganker is less needed, so you could use either Kallari, Rampage, Feng Mao or Sevarog jungling. I would personally recommend either Kallari or Rampage. In a 2-0-2 setup, you push the outside lanes with a carry and a tank/support on both sides. You rely solely on a ganker to protect the middle lane. The goal of this setup is to trick the enemy to overextending to the middle, at which point you push hard on the outside and try to take the inhibitors. Since Kallari is definitely the best ganker in the game, he should be used to most effectively gank the middle. Laning, I'd use Steel+Gideon one one side and Muriel+Murdock on the other
3 Laners, 2 Junglers
With 3 laners and 2 junglers, you have 2 pushing options. One is to go 2-0-1, which uses one carry on each side lane with Muriel as a flex support character. The other option is to go 1-1-1, with a carry in each lane. This is the more defensive option, as using this strategy is basically relying solely on your junglers being better than their junglers in the late game. For both of these, I'd use Sevarog and Kallari jungling, as they both can gank fine and Sevarog can stack. However, if you are unimpressed with Sevarog in the 2-0-1 formation, you could use Twinblast as a better ganking replacement. In 2-0-1 you should use Murdock and another carry with Muriel as a flex character. In 1-1-1, you should have Murdock and two other carries holding the fort.
Counter Composition
Counter composition is a strategy where you adapt to defeat the opponent's strategy. It uses flexible heroes that fill many roles to be able to effectively adapt. Against a 3-0-2 rush strategy, your best bet is to counter it with 2-0-2 and rely on your Kallari to effectively gank the other team's carries and support. This will allow you to overpower them in the early game and sweep them in the late game. Against 1-2-1, you want to bring a 2-0-2 strategy, relying on Kallari to gank mid effectively and overpowering them on the sides. Against 2-0-2, you want to bring a 2-0-1 squad with two junglers, who you're relying on to gank well. Against 2-0-1, you can just overwhelm them by bringing a 3-0-2 and crushing them on two fronts. Against 1-1-1, sit back in a 2-0-1 and try to push against them. From these strategies, you need to formulate a "one fits all" team. You'll be using a 2-0-2, a 3-0-2 and a 2-0-1. Since we'll need two gankers, one of which needs to be good at another position, you'll be bringing Twinblast and Kallari to gank. Muriel is a given because Muriel is awesome. You'll need a tank on the other end, so Steel is the choice. For the fifth character, I'd use Gideon as a solid all around character with great Steel synergy.
TL;DR- Read Fitting Heroes into Compositions.