Let's start here: when I refer to a sump pump, I am specifically speaking about a sewage lift pump. They are typically found in a basement of a house where the city sewer or the septic tank inlet is higher than the floor of the basement. They are always installed in what we usually call a "barrel." It's exactly like you would think: a large, sealed (usually plastic) barrel that houses a pump inside. They are often roughly 24'' in diameter and roughly 36'' deep. From a homeowners perspective, only the top will be seen as they are installed under the floor to allow for basement plumbing to drain into them via gravity.