lhs :> rhs or lhs rhs represents a rule that transforms lhs to rhs, evaluating rhs only after the rule is used.
The character can be entered as :> or \[RuleDelayed]. You can apply rules using Replace. The assignment lhs := rhs specifies that the rule lhs :> rhs should be used whenever it applies. You can use Condition to specify when a particular rule applies.
lhs rhs can be entered as lhs \[RuleDelayed] rhs or lhs :> rhs. In most cases, RuleDelayed is printed using . See notes for Rule. See The Mathematica Book on the web: Section 2.4.1 and Section A.5.5. See also: Replace, SetDelayed, Rule.