Basically, intelligence is how many turns an alien will remember a soldier that it has seen previously. The higher the number, the longer an alien can remember a soldier (an alien who remembers a soldier knows the soldier's exact position, so can target the soldier even though it can't see the soldier, throwing a grenade through smoke or moving to just the right spot to get the perfect shot).
I am not sure if aliens remember soldiers that other aliens have seen as well, but there is a shared awareness, at least when it comes to psi-attacks. The alien doing the psy attack doesn't have to be the one who spotted the target.
Aggression is just how likely an alien is to risk itself to kill a soldier. Chryssalid for example are very aggressive, so they will rush your soldiers even though that might leave them exposed. I am not sure which is the least aggressive, but sectoids always seem a bit skittish to me, taking shots and then getting back into a hiding spot.