For example, the difference between "lvldwarvenspider" and "lvldwarvenspiderambush"
Do they only come out when the player gets really nearby?

Yeah Ambush enemies are the ones that are 'hidden' until the player approaches or they are triggered. Like your two examples; the normal one would already be standing there crawling around. The ambush one could be placed in a pipe so it pops out when the player comes near it.

Another example is the Draugr that are laying down in the crypts, that stand up when you come near them. Those are 'Ambush'.

Okay, thats what I figured. Unfortunately, they aren't acting much like ambush enemies in my mod. If I fight something in one part of the dungeon they all come running. I may have to set up triggers that spawn them.

You do have to mate. Triggers to enable them. These will help:

http://www.creationkit.com/Bethesda_Tutorial_Traps_and_Prefabs

http://www.youtube.com/watch?v=PN5vCtlxvwk&feature=youtu.be&hd=1

That's a common problem with large quantities of Ambush Enemies. It is a good idea to make sure that - once you set them to some sort of trigger (whether it be a trigger volume or not) - that you check the "Parent Activate Only" box on the references "Activate Parents" Tab.