For starters, champs have a symbol in the top right of their thumbmail, this indicates their aura skill. you can see the details under their skills tab.
Aura is an ability bonus that the whole team gets, but only the leaders aura is active.
If the leader's aura is a white shield on a green backgrond, everyone gets a defense bonus. If it's a white heart on a blue background, everyone gets an hp bonus.
Note that not all bonuses apply everywhere; some will only work in arena, some will only work in dungeons, some work everywhere, and so on.
For an example about the order skills are used in: You want to make sure, or at least try, to get debuffs landed before you do your biggest hits.
if your nuker is the fastest champ on your team, you don;t want to open with the grand slam, so do an a1 attack instead, then let someone else put defense debuff and weaken on cb, THEN let the nuker make the big spectacular hit.
Complex speed-tuned teams can require an incredible amount of precision to make sure everything goes off in the correct order on auto.