Compressors can really make or break a sound, a vocal or an entire track. We are talking about dynamic compressors, not software programs which sample a larger digital file in order to shrink it. (AIFF to mp3, for example.)
Fairchild 670, Fairchild 660, Neve 2254 and Neve 33609, SSL and api 525s are some of the classic compressors, each with their own sound. Compressors can be used on a vocal chain, any instrumental tracks and even a stereo pair on a mix buss.