Welcome to the fight!
Anyways, there have been some massive changes into the run up to the Ancient Shadows release to how the AI attacks you. I'll try to run things down.
(Everyone else chime in also, I'm going to use this thread to update the wiki once it has run its course.)
Border AggressionThis still exists in the game but with the other mechanics I'm going to list below, it is now very, very rare that this mechanic gets invoked.
System DefenseCalled "Basic AI Reinforcements" on the wiki, these are pretty much unchanged from what the wiki says. The only difference I noted was that it is no longer true that it only reinforces planets on alert until they are full. It certainly still prioritizes planets on alert, but there is a decent chance now that other planets, especially high-value ones, will get a reinforcement wave.
Also note that this is where Barracks come from if they appear after the game starts. As the number of ships on a planet starts to get really, really high, the AI puts them in barracks. The catch here is I am not sure if ships inside a barracks count towards the planets ship cap or not. I think they do? But I'm not actually sure.
ThreatThreat is freed ships that are sitting around waiting for the chance to attack you. They tend to cluster on the far side of wormholes from your territory. Threat comes from sources such as the survivors of an AI wave that ran away or defenders of a Guard Post you have destroyed.
If after about 30 minutes ships on threat don't see a chance to attack, they change to ThreatFleet and fall back to a rally point to join up with other ThreatFleet ships that look to attack you at a future point in time.
Ships on Threat and ThreatFleet do not count towards a planet's ship cap for reinforcement purposes.
Special ForcesSpecial Forces are ships the AI holds in reserve that it uses to defend high-value planets. (Planets with an ARS, Adv Fac, Core Fab, etc.) They are deployed in response to a players attacks so their is a small window after the player ships warp in before the Special Forces ships will warp in.
If the Human forces retreat, the Special Forces will return to base after a short while.
Destroying a Special Forces Guard post (and incurring the AIP cost) will decrease the size of the Special Forces Reserve by 5%.
Strategic ReserveEssentially the same as Special Forces, but only deploys to defend the AI homeworlds and Core worlds. (Core worlds being those adjacent to an AI Homeworld.)
There is no way to decrease the size of these and they tend to be bigger as their first deployment is not until late in the game when the Player enters the Core worlds.
(When I do the wiki I'll see if I can get exact numbers out of Keith, but this is a start.)
D.