For[start, test, incr, body] executes start, then repeatedly evaluates body and incr until test fails to give True.
For evaluates its arguments in a non-standard way.
For[start, test, incr] does the loop with a null body.
The sequence of evaluation is test, body, incr. The For exits as soon as test fails.
If Break[ ] is generated in the evaluation of body, the For loop exits.
Continue[ ] exits the evaluation of body, and continues the loop by evaluating incr.
Unless Return[expr] or Throw[expr] is generated, the final value returned by For is Null.
Example: For[tot=0; i=0, i < 3, i++, tot += f[i]]. Note that the roles of semicolon and comma are reversed relative to the C programming language.
See The Mathematica Book: Section 2.5.9.
See also: Do, While, Throw, NestWhile.