expr /. rules applies a rule or list of rules in an attempt to transform each subpart of an expression expr.
Example: x + 2 /. x -> a .
ReplaceAll looks at each part of expr, tries all the rules on it, and then goes on to the next part of expr. The first rule that applies to a particular part is used; no further rules are tried on that part, or on any of its subparts.
ReplaceAll applies a particular rule only once to an expression.
Example: x /. x -> x + 1 .
See the notes on Replace for a description of how rules are applied to each part of expr.
expr /. rules returns expr if none of the rules apply.