The best explanation I've ever seen is A Brief History of Time by Stephen Hawking.

It's an awesome book, and explains quite a bit about general relativity.

As for what spacetime is, it's, well, space and time. The two things are the same is the gist of it. Time is a dimension, just like height and width.

There's more to it than that, but Sir Stephen does a much better job of explaining it than I ever could.

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"Science is the poetry of reality." ~ Richard Dawkins