Siding is what a home builder covers the outside walls of a house with. Some houses have old aluminum siding or newer vinyl siding.
Older houses that haven't had new siding applied are likely to have some type of wooden siding, like shingles or clapboards, unless they're made out of brick or stone. It's less expensive to maintain a house with plastic, or vinyl, siding because it doesn't have to be painted every few years. A completely different kind of siding is a track that runs beside the main train tracks, used for storing train cars or allowing one train to pass another.