An injunction is an order by a court commanding or prohibiting action. An injunction usually is a court order demanding that one party stop doing something that is damaging to another party. For a complainant to receive an injunction, the complainant must show that no other remedy for the wrong is adequate or complete.
Injunctions are often used in non-compete situations where one party is believed to be violating the non-compete agreement and is thus damaging the other party. The complainant will attempt to get an injunction to stop the competitor from creating additional damage, until the case can be heard. This type of injunction is sometimes referred to as a temporary injunction or a preliminary injunction.
Injunctions are also common in copyright or patent violation situations, for the same reason - to prevent harm to the copyright or patent holder until the case can be heard.
Injunctive relief is a term used in contracts (non-compete agreements, for example) to indicate that one party will seek "injunctive relief" (that is, will seek an injunction) against a suspected competitor.