What exactally do fleets do?

Fleets allow for much easier use of a group of ships, and also allow them to be more effective in an attack, as they can jump at the same time.  Putting ships into a fleet enables them to act as a coordinated whole, instead of however many individual ships.  like putting a trained football team on a field against a random group of people who you just tell to "play football".  the trained team is going to win unless greatly outnumbered everytime.  its just group cohesion. 

in a fleet, units act in a more organized way - combat ships engage the enemy from one direction, and protect the support ships, support ships focus on helping allied ships, and caps do both.  when not in a fleet, support ships attack the enemy, caps charge right into the fray, and combat ships engage from different sides and in small groups, allowing them to be easily overwhelmed if focus fired upon.

that should help.

Fleets insure that all ships stay in a formation. This means that as many ships as possible will all be firring at the same time. Instead of having each ship fly around on it's own and when you give the order to fire they each fire when ready, a fleet will be all firing together as much as possible.

There is a big disadvantage to this which is speed. Fleets can be very powerful in certain situations but if the enemy can stay out of your range then he can safely pick away at you.

Fleets are most useful assaulting defended posititions. You know you're going to take damage from the defenses and you want your forces to do the most damage possible while receiving the least damage necessary.

In general combat fleets are more of a burdon. I rarely use them because it makes all yoru ships move so slowly.

I'm not sure if there are bonus's built into fleets? Maybe i'm just not aware of a boost to some stat that fleets get. The loss of mobility is a big deal in most games I play. I'm going to play with fleets, maybe I'm not using them effectively.