What are Bilinear and Trilinear ??

They are texture filtering methods.

Trilinear uses more levels of dithered filtering than Bilinear, so it is better.

Both are inferior to Anisropic Filtering, however. But of course, Anistropic filtering gives the biggest performance hit. (although it's not much of an issue in the mid and top end cards)
 
Right, and they are performance killers just like the AA and AF so use the bilinear for optimum gaming with performance and quality or not.

Right now with my SLI 7800GTX I used the Bilinear with 4xAA 4xAF and Multisampling. This is my setup for my BF2 online gaming.
 
Ya, same. Now that I was playing some demo (can't remember which one) I noticed that you couldnt select Bilinear / Trilinear when AF is on.