There are basically two types of bristles:
Flocked and monofilament. Flocked means that the ends of each bristle are slightly crushed giving a softer tip. Monofilament means that each bristle is a single strand with a straight cut off end.
You can then have different styles of brush.
Single (full) trim and dual trim. Single trim means that the bristles are all one length. Dual trim means that there are two different lengths of bristles on the brush, usually shorter ones in the middle of the brush.