This is a basic overview. As you study these terms in more detail it should all become more clear.
Most of the AI must have an AI Volume to function within. This basically tells the AI where they can go. Thus AI vloumes should not pass through walls and geometry like crates and vehicles, cause then an AI will get stuck there.
Nodes are more specific points within AI Volumes that give specific purpose to the AI.
Example: "At this point, you will crouch down and take cover from the player shooting at you"
AICOVERNODE is placed behind a crate,
AI SNIPE NODE
AI PATROL NODE
AI ALARM NODE
You would probably want AI to behave differently. Some may attack, while others take defensive positions and snipe from cover. You use goals within the AI's properties tab to differentiate and depending of the goals the AI use different Nodes. Conceptually pretty simple.
The actual Character choices influence the AI's behaviour as well. Most Corporates have a better interaction pattern then do the Marines in the original AvP2 game. That's because they were the enemy for the player, and coded. Most the Marines were merely used in Cut Scenes and there involvement was more scripted. Characters are given goals and brains and there are a multitude of options to make them quite interactive.
Why does AvP2 not have BOTS?
One its not coded in the source. The AI could actually behave like deathmatch bots, they already have significant interaction so it is possible, regardless what some un-educated people might say. AI's are coded to handle themselves fairly well with the existing code. Nonetheless they do need some tweaks. LAST ONE OUT has included a couple DEATHMATCH scenarios to prove that the AVP2 AI is quite capable of holding its own. Unfortunately without the source they can not be put into existing MP maps using some menu setup, but rather included in SP skirmish maps only.