Well, there is no easy definition of a crate motor. It's easy to say that a crate motor is a brand new engine assembly, usually with a warranty, delivered right to your front door. And though that's true, things don't stop right there.
Some crate motors are just an engine block, crankshaft, and pistons, all nicely bolted and torqued together. This is called a short-block. A long-block crate motor is a short-block, but with the cylinder heads and gaskets also in place.
Next up the menu (And price range) is the more or less complete engine with all of the above, plus an intake manifold and exhaust headers. And finally, there is the ready-to-run option that includes everything you'll need except for oil, fuel, and electricity.
Oh, and money. You'll need money. Most crate motors are expensive. Really expensive. Why, a new GM 640-horsepower Supercharged LS9 like they drop into a Corvette will set you back around 30,000 big 'uns. But the ease of just dropping in one of these power plants saves a lot of time which makes up for a lot of that expense. Or so "They" say.