A masonry chimney is brick or stone, with a clay flue tile sticking up through a concrete crown. It takes a chimney cap: a metal lid on legs with a mesh screen around it, fixed over the flue. A chimney with one flue gets a single-flue cap. A chimney with two or more flues is often better served by one larger cap that covers the whole top.
A factory-built (prefab) fireplace vents through a metal pipe inside a framed, sided chase. The top of that chase is sealed by a chase cover, a sheet-metal pan with a hole for the pipe. Rust streaks running down the siding below the chimney top are the classic sign that the cover has failed and needs replacing.
Material matters. Galvanized steel is the cheapest option and the one that rusts. Stainless steel costs more and holds up far longer, so it is usually the better buy for both caps and chase covers.
Caps are not one-size-fits-all. Flues come in many sizes and shapes, and a cap that doesn't fit is a cap that blows off. We measure on site, check the crown the cap will sit on, and clear any nest or debris from the flue before the new cap goes on. If the old damper is rusted out, a top-mounted damper that seals the flue at the top can do both jobs.
If the visit turns up a cracked crown or failed flashing, that is covered on our chimney repair page.