Necessary -- something is necessary to bring about an action (e.g., it is necessary that a soccer ball must be hit toward a goal for a point to be scored). Note, however, that you can kick a soccer ball toward a goal and still miss/not score, so necessary events don't always bring about an outcome.

Sufficient -- something is sufficient to bring about an event (e.g., successful completion of all degree requirements is sufficient to achieve a degree). This is because X (completion of requirements) always results in Y (achieve the degree).