I can't seem to find a source for an explanation of what each clip texture is good for. Has anybody seen one? Much like the excellent Radiant tool bar explained I've seen here on ModsOnline.
I think there's a lot of magic tricks hiding in there.

The clip textures change between games - their names and appearances that is, so I'll attempt to explain the ones primarily in COD4 that are needed the most (brackets = what the texture looks like):

Clip - (CLIP): Players and AI will collide with this like a solid wall. If this is between two path nodes it will also cut the path.

Clip_ai (MON): Only AI can collide with this, it will break the node path as well but players can walk through.

Clip_foliage (FOLIAGE): This makes the sound of foliage when touched by a player but I'm not sure if it stops them or if AI touching it can generate the sound.

Clip_full (FULL): Blocks players, AI and projectiles.

Clip_magic_jeepride (CLIP NOSIGHT CLIP): This was used in the last level instead of caulk on the Bridge destruction scene, I don't know it's exact properties - there's a whole bunch of these.

Clip_metal (CLIPMETAL): Sounds like metal when shot, walked on etc and stops players

Clip_Missile (MISSILE): Stops projectiles only

Clip_NoSight (NoSIGHTCLIP): Prevents the AI from 'seeing' past but does not block them. (aka 'blind spots').

clip_player (Player): Stops the player like a solid wall, but allows AI to pass - does not affect node/paths.

Clip_weap (WEAP): stops all bullets from passing

Clip_Weap_glass (CLIPWEAP GLASS): Reacts like glass when shot and bullets won't penetrate. (I think that was the original idea but in actuality bullets do go through from what I remember).

Basically a lot of them are self explanatory but when in doudt, make a test map and stick them in it

Wow, I didn't expect all that in a reply! Very appreciated and helpful!

Is there a clip that can keep foliage, i.e. shrub branches, from penetrating a wall or other object that is sits near? I have some shrubs in my map that protrude through a wall so I had to move them farther away, but it doesn't look as good. I couldn't find a neater boxwood type of bush so had to use the wild looking shrub.

Thanks.

Unfortunately Models are what they are you can't edit them OTHER than scaling which could help you.

Select your foliage model and hit "N", type:

key: modelscale
Value: 0.5

(So this will half the size of the model, other decimal and whole values can be used too)

So now you can place small foliage near your building but it's a good idea that if you decide on making a building with vegetation close by then make the walls thicker... OR build the foliage into your design but destroying walls etc to make it look overgrown.

A FOLIAGE brush doesn't block the player (or the AI, probably), but for me doesn't create any sounds either.

As with clip_metal, the other clip_nosight textures create the sounds for other surfaces (wood, rock, snow etc).

im not sure but the foliage might work only in single player, otherwise ive never noticed it myself

also anyone else had the clip_weapon block the player?

In COD4 the Foliage does create leaves/bushes moving sounds as the MP player passes through them in the condition that your level contains the correct Soundaliases call in the Zone File:

sound,common,mp_yourmap,!all_mp
sound,generic,mp_mp_yourmap,!all_mp
sound,multiplayer,mp_mp_yourmap,!all_mp

Is mp_mp_ correct or you copy/pasted too much ?

This is what I have in my zone file, sound related:

sound,common,mp_bhd,!all_mp
sound,generic,mp_bhd,!all_mp
sound,voiceovers,mp_bhd,!all_mp
sound,multiplayer,mp_bhd,!all_mp