photoshop is mostly used for photo manip illustrator is a vector tool its based off the path tool and stuff.

dont know main differences but the professional designers use illustrator too and i only know that illustrator is vector based program dunno if ps too it depends on the quality of your work

What program do people use for Fancy UserTiles and Texturing?

photoshop and gimp ik atleast 10 people who use illustrator for texturing

photoshop,gimp,illustrator whichever they want

What is the best program in your opinion?

gimp is easy to use and can give you quality stuff once you use to it so imma go with gimp.