A decision includes if statement, loops, Case statements etc.
Decision Coverage measures the coverage of conditional branches; branch coverage measures the coverage of both conditional and unconditional branches.
A weakness of the decision coverage test technique pertains to the fact that the tests performed are inadquate for high-integrity applications, because they do not ensure decision coverage of the object code. Additionally, an incorrect evaluation of a condition can be masked by other conditions. For example, logic errors are not necessarily visible. [Source]
100% decision coverage always means 100% statement coverage.
Also See:
Statement Coverage