One of the most important of the provisional remedies is the injunction-- a court order that prohibits a party from doing something, or requires him to do something. In the former instance, the injunction is called a prohibitive injunction; in the latter instance, a mandatory injunction.
An important purpose of the injunction is to prevent the litigation from becoming an academic exercise. To take an egregious example, suppose a convicted felon is on death row, and his lawyer is appealing his conviction. If the felon is executed while the appeal is being considered, the appeal becomes a meaningless exercise. To prevent that, the felon's lawyer will seek an injunction barring the state from executing his client during the appeal.
In some cases, the injunction bars conduct while the merits of the litigation are being adjudicated, in which case the injunction is called a preliminary injunction. (In the example of the felon on death row, the preliminary injunction bars the execution while the appeal is prosecuted.) In other cases, the injunction bars conduct after the merits have been adjudicated, in which case the injunction is called a permanent injunction. (In the example of the felon on death row, the permanent injunction, issued after the conviction is overturned, permanently bars his execution.)
The preliminary injunction is a remarkable thing. A pleading is just a set of allegations that have to be proved. The preliminary injunction affords relief before anything has been proved and adjudicated. To lessen the unfairness to the party enjoined, a federal court requires a showing of two elements: (1) the likelihood of irreparable injury in the absence of an order or injunction; and (2) either (a) the likelihood of success on the merits or (b) sufficiently serious questions going to the merits to make them a fair ground for the litigation plus a balance of hardships tipping decidedly in the moving party's favor.
So take the example of the felon on death row. With respect to the first element, his irreparable harm-- harm that can't be repaired-- is that he's going to be put to death. With respect to the second element, he may be able to show that his appeal is strong. If not, he must show that the appeal raises serious questions, and of course, he can show that, because he'll be put to death, the balance of hardships tips in his favor.
The irreparable harm element is generally considered the harder of the elements to show. Irreparable harm is an injury that is not remote or speculative but actual and imminent, and for which a monetary award cannot be adequate compensation. Examples of irreparable harm include damage to a valuable trademark by the sale of knockoffs; harm to a business reputation; emotional or physical harm; and the destruction of an ongoing business. In such instances, it may appear that the harm can be remedied by a money judgment, but the difficulty of quantifying the harm is what makes it irreparable harm.
Obtaining an injunction can be of great tactical importance. Suppose, for example, that the plaintiff is able to enjoin the consummation of a business deal that his adversary is party to. Getting the injunction may cost the adversary so much money that, as a practical matter, the injunction brings the litigation to a close, sparing years of costly litigation.
Of course, it should be mentioned that the person obtaining the injunction can be sued for damages if it proves to have been wrongfully obtained. And typically, the person obtaining the injunction will be required to post a bond that ensures that there will be assets with which to pay the person enjoined in the event that the injunction is proved to have been improper. Nonetheless, having the skills for obtaining an injunction is an important part of a litigator's weapons.