There is some instability in the terminology here. Many authors use Reductio Ad Absurdum (RAA) as meaning the same as proof by contradiction and indirect proof. More careful authors distinguish them, taking both RAA and indirect proof to be a species of proof by contradiction.
In what follows, I use P and Q for propositional meta-variables, ∧ for conjunction, ∨ for disjunction and ¬ for negations. (I do wish for LaTeX!)
RAA proceeds by assuming some proposition P, on that basis deriving some contradiction such as Q ∧ ¬Q, and, having reduced P to absurdity, concluding ¬P. In the context of a natural deduction proof system for logic with introduction and elimination rules for each connective, this is ¬-Introduction.
Indirect proof is the very similar method of proof whereby you assume ¬P, derive a contradiction such as Q ∧ ¬Q and then conclude that P. In the sort of natural deduction presentation just mentioned, this is the rule of ¬-elimination.
These two principles are very much worth distinguishing. If you take a classical propositional logic natural deduction proof system with introduction and elimination rules for each connective and remove ¬-elimination, the result is intuitionistic logic. This logic is more often thought of as being characterized by the denial of the law of excluded middle (P ∨ ¬P), but the two characterizations are equivalent for propositional logic.
Intuitionistic logic is one of the best studied and oldest non-classical logics, and one which plays a prominent role in many debates in metaphysics.