Scheme (as in the R5RS spec and earlier) is a very minimal language specification. This means that all practical Scheme implementation add features not present in the specification. In some areas the Scheme implementors agree - and in others: not so much. Racket has many features not covered by R6RS/R7RS. However due to the module and macro system that enables you create languages (that compiles to Racket modules), it is possible for Racket to provide a language compatible with Scheme. That is: Racket includes an implementation of Scheme.

It was, but then diverged from r6rs and became it's own. One big things is that the cons cell is immutable by default (which means immutable lists), where scheme has set-car! and set-cdr!

The racket team wrote a bit about it here: http://racket-lang.org/new-name.html

The Racket language does not follow any of the Reports, so it's technically not a standards-compliant Scheme. On the other hand, the Racket platform supports both R5RS and R6RS out of the box, and there's a library for R7RS support.

They're doing a lot of research, so their personal views on how to leverage lisp/scheme heritage shows. For instance, they go deep into eDSL.