A brief discussion of lingo and what can kill you:
SF: Special Forces. Independent fleet that will defend CSGs and other 'important' items under the AI's control. Can get pretty massive.
CPA: Cross Planet Attack. I just rewrote the wiki page for these but basically a massive wave that's sent at you from existing forces + homeworld defense reserves. wiki page here:
http://www.arcengames.com/mediawiki/index.php?title=AI_War_-_Cross_Planet_AttacksCBA/Border Aggression: This is from a planet overpopulating due to reinforcements. Short version: A planet will max off at ~500 ships if you leave only the cmd center + wormhole guardposts, and at that level will never border aggression you. This is called neutering.
Waves: Waves are spawned via warp gates on neighboring planets (initially) or far-off gates if there are no warp gates adjacent to your territory. If you use a single whipping boy (removing all warp gates but one against your territory) then the AI will use larger waves by waiting longer to build the wave. If you have multiple ingress points you have a *chance* at lower timed wave buildups.
Reserve Forces: Come out and smack you upside the head when you get near the AI HW.
Threatfleet: A fleet that is built from roaming threat after said threat has been idle for 30 minutes camping on your doorstep. This threatfleet will hang around 3-4 planets out and will constantly assess if they can find a hole in your defenses, and then work their way to you. Side note: Apparently multiple threat fleets are possible, if you cut them off in different clusters.
... think that covers the main ones.
[Edited for typo]