what are the team member tasks really do ?
 

Does anybody know what happens when I set on member of my wing to "defend"? And is there a difference if I set someone to team leader instead of attack? And what are the benefits from putting 2 people in one team as team leader and attacker instead of having 2 teams with a team leader each?

A team is the primary element of tactical combat.
Assigning a team leader, pay attention to character‘s
skills It is equally important to select the right
supporting ships and set appropriate objectives:
attacking, defending, or antimissile defence .
By adjusting objectives and even replacing the team
leader during a battle, you can fine-tune the team
depending on enemy types and numbers, weapons
and special equipment .
You can make changes to your team during a battle
in the tactical pause mode (press "space") Combat
is ever-changing, so a team that was effective
at battle start might
turn into an easy prey
later . To assign a team
leader, drag a pilot
portrait onto the leader
icon .To create a team,
drag a supporting
teammate icon onto
the leader icon .
Supporting teammates can perform the following
tasks:
Attack - A teammate will attack leader‘s target
Defense - A teammate will only return fire
to enemy ships attacking the team
Repair - A teammate will focus on remote
repairs of all team ships (provided that he
or she has a remote repair system installed)
AMs - A teammate will use an antimissile
system to shoot down incoming missiles
stealth - A teammate will use stealth systems
to conceal the team from enemy scanners and
launch surprise attacks

Thanks for the comprehensive answer. Normally I choose the character with the tactician skill as leader. But if noone has this skill I don't see a point why one pilot should do better as the team leader than the other. So where is the trick in that?

Hmmm the pilot with good piloting skills, i always use the hero... i always chose piloting specialization for the main character... next one\two should be gunner/sniper etc...