The Return has all of the same characters: Elena, Stefan, Damon, Bonnie, Meredith, Matt...They're just on a different adventure. It also involves some weird Japanese twins, snow globes, and magic wings, but I wont get into all that.
I suppose it's called "The Return" because it wasn't part of the original series and was written years after the first VD books. The original series needs to be read first, though. I know some people that read The Return first and they were so lost. The book is confusing enough as it is. But honestly, you don't want me to start ranting about it because once I do, I'll never stop. But yes, to answer your question, The Return is a sequel.