Yachts can be both with and without sails and they vary in size. What makes a yacht a yacht is that it’s a recreational boat or ship - it’s used for racing or pleasure.

Mikkel's right that the definition of a yacht is a recreational boat or ship. The Royal Yacht Britannia is massive luxury ship, 125m long, but it counted as a yacht because it was used for recreational purposes. By the way, forgive me for stating the obvious....but are you sure that the pictures you saw weren't just sailing yachts with their sails and masts down?

It can be either. There are motor yachts and sailing yachts, but usually we simply call them all 'yachts'.