This is a great online book regarding Scheme - in particular, this opening statement about Scheme is one of the best encapsulating paragraphs I can find:
Scheme is a lexically-scoped, block structured, dynamically typed, mostly functional language. It is a variant of Lisp. It has first-class procedures with block structure and indefinite extent. Parameter passing is by value, but all values are references. It has first-class continuations to allow the construction of new control abstractions. It has lexically-scoped ("hygienic") macros to allow definition of of new syntactic forms, or redefinition of old ones.
The book can be found on http://www.federated.com/~jim/schintro-v14/schintro_toc.html or on ftp://ftp.cs.utexas.edu/pub/garbage/cs345/schintro-v14/schintro_toc.html