The term small press can be interpreted a number of ways, from non-profit-making one-man-band publishers, to publishers with several employees. Regardless of size, they are a valuable outlet for writers, especially those seeking with few or no publication credits or whose work does not fit in with mainstream publishing trends.
Small presses should not be confused with vanity publishers, which often masquerade as larger presses. Most small presses do not pay contributors, except, in most cases, with a complimnetary copy, but only a few charge reading fees and these are best avoided. With many accepting email submissions, you may not even need to pay the costs of postage to submit to them.