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].
RuleDelayed has the attribute HoldRest.
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 StandardForm, RuleDelayed is printed using .
See notes for Rule.
See The Mathematica Book: Section 2.4.1 and Section A.5.5.
See also: Replace, SetDelayed, Rule.