So far, all the commands that you've shown are calls to procedures. Is that basically what a command is Scheme, or are there other kinds of commands?

A Scheme command always directs Scheme to evaluate an expression. However, there are many kinds of expressions in Scheme -- procedure calls are only one of them. You've already seen numerals, which are another kind of expression. You can command Scheme to evaluate a numeral if you like, but in the case of integer numerals the results are pretty boring:

> 23
23
> -7462
-7462
> -0
0

An expression is a syntactic construction that can be evaluated to obtain a value. Every expression in Scheme is of one of the following kinds:

1. Variable references
2. Literal expressions (including numerals)
3. Procedure calls (sometimes called applications or combinations)
4. Lambda-expressions
5. If-expressions
6. Assignment expressions
7. Cond-expressions
8. Case-expressions
9. And-expressions
10. Or-expressions
11. Let-expressions
12. Let*-expressions
13. Letrec-expressions
14. Compound expressions
15. Do-expressions
16. Named-let-expressions
17. Quasiquotations

The first six kinds are collectively called primitive expression types; the rest are derived expression types. Derived expressions are theoretically superfluous, since they can be translated into equivalent expressions of the primitive types; in fact, many implementations of Scheme perform such translations before evaluating derived expressions. In most cases, however, derived expressions are easier for human readers to interpret and understand than their primitive equivalents.

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http://www.math.grin.edu/~stone/scheme-web/commands.html

Copyright 1995 by John David Stone (stone@math.grin.edu)