Preliminary Injunctions vs Temporary Restraining Orders

There are, for sure, similarities between preliminary injunctions and temporary restraining orders. The differences tend to show up a bit more on the MBE, but first it's important to understand some commonalities:

Both temporary restraining orders ("TRO") and preliminary injunctions are types of injunctions. An injunction is an equitable remedy by which a person is ordered to act or ordered to refrain from acting. Injunctions are often granted while a case is still pending (sometimes referred to as "interlocutory injunctions") and are often granted to maintain the status quo until a trial on the merits is completed. 

Temporary Restraining Orders: There are some strict requirements when seeking a TRO. First is that waiting for a preliminary injunction is not feasible since irreparable injury will occur before the hearing on a preliminary injunction. Generally, the adverse party (the party not seeking the TRO) must be given oral or written notice of the TRO hearing, but not always. A TRO may be imposed without notice of a hearing if the moving party (the party seeking the TRO) states specific facts in an affidavit or verified complaint of the irreparable injury that he/she will suffer if the TRO is not granted. In addition, the moving party must certify in writing the efforts made to notify the adverse party as well as the reasons why notice should not be required. Lastly, the moving party must provide security to pay for any damages incurred by the adverse party if the court later finds that the adverse party was unlawfully restrained. 

A nuanced point to keep in mind is that although a TRO can be issued without notice (if those requirements above are satisfied), a person must have actual notice before the person can be held in contempt for violating the TRO. 

As for timing, a TRO lasts for 14 days. It may be extended for another 14 days, but no longer than that. In other words, 28 days in total, at most.

Preliminary Injunctions: As with a TRO, a preliminary injunction is meant to preserve the status quo before trial. Unlike with a TRO, notice is required. The adverse party must be given notice and an opportunity to be heard. A preliminary injunction commonly will be granted if the following conditions are satisfied: the plaintiff will suffer irreparable harm if the injunction is not granted; the harm to the plaintiff outweighs the harm to the defendant; the plaintiff shows that he/she is likely to be successful on the merits; and the public interest favors granting the injunction. 

Measuring irreparable harm can be tricky, but if the party seeking the injunction has an adequate remedy at law (look for damages) then it's highly unlikely that the condition of irreparable harm is satisfied.