Something that is similar to talking shop. Small talk is usually less codified, less exact, and less intense. It is casual. It can change to polite conversation if one of the conversants decides that they don't like one of the other ocnversants.
Small talk is kind of in between polite conversation and talking shop. It's when you sit around, and, for lack of anything better to do, you talk to the people around you about nothing in particular, as opposed to talking about salient issues or something really intellectually stimulating. The classic topic of conversation in small talk is the weather.