A zoom lenses is one which can change focal lengths, as opposed to a prime which has a fixed focal length.
A telephoto is taken to be a lens with a long focal length and can be either a zoom or a prime.
Generally anything from 70mm onwards on a 35mm camera would be called a telephoto, which would make a 50mm 'normal' lens a short telephoto on most DSLRs, though some people only using the term for lenses with longer lengths such as 200mm and beyond.
Technically though a telephoto lens is one that has been constructed in a particular way to allow it to have a shorter size than its effective focal length, usually making it about half as long as it would otherwise need to be.
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