If you payed attention to the episode where Rei got invaded by the string angel and the episode where the space angel mindrapes Asuka, you can pretty much get a good idea what they are about.
Basically, Angels all have their own personality and vice. The angels that don't talk and 'infect'(the episode with the Magi infection) are trying to learn about man to destroy them. The rest are just trying to destroy man to earn god's favor.
If you follow the metaphor. If you don't its just a bunch of aliens trying to kill the humans or save their friend that got stuck on the final level. Either way, mindfuckery.
The only difference between Eva's enemies and your standard mecha anime enemies is that they gave them angel names and they couldn't always be fought with the mechs.