A 30-day free trial gives you access to a game or service for a fixed, limited amount of time without having to pay for it until the time is elapsed. After that, the trial period ends, and you'll have to dish out the dough if you want to continue. If it's a subscription-based service, you'll often be billed automatically for another month once the trial is over.
If you want to cancel your subscription before the trial period expires, it's not unheard of for the vendor to make it difficult to actually cancel the service. They already have your credit card number, and may continue to charge you for services you no longer want until you go through the "proper" procedures, or dispute the transaction through the credit card company. In most cases, this only happens with obscure companies, but some exceptions have occurred.  
Thirty days is common, but any length of time counts.
Not to be confused with the Freemium model, where the free version of the game has limited content, but no time restrictions; Freemium services often include a 30-day free trial of the premium version. Shareware often uses this model as well.
If advertising plays up the trial version with phrases like "Play now for free!" while playing down the cost, it may be an Allegedly Free Game.
- Free players of RuneScape can try a 14-day free trial of membership without having to pay.
- EVE Online has a 14-day free trial, but there are ways to get a 21-day trial.
- Star Wars Galaxies offered new accounts a 14-day free trial of the game, rewarding those who subscribe at the end with a bonus item that boosts their Experience Points temporarily.
- Rift had a seven-day free trial, until it went Free-to-Play.
- Most of the games at Big Fish Games have a one-hour trial period. If you want to keep playing after the hour, you'll need to buy the game. (Since they're a Casual Video Game company, not an MMO, one hour is usually enough time to see if you like the game or not.)
- GameHouse, another Casual Video Game site, works the same way as Big Fish Games: The games can be played for one hour free; to play longer you need to buy it.
- Their FunPass has a 2-day trial.
- WildTangent works in a similar fashion to the two websites above.
- PlanetSide once had a Reserves event (where players were limited to level 6 (out of 25 at the time) and Command Rank 1 (out of 5)) which included a one year long trial that stood up to its name by attracting a lot of players. The game also had a seven-day trial before and after the Reserves, but a few years after the Reserves ended, the trials ended because they were very convenient for hackers - they'd get banned, then immediately make another trial account.
- America Online's crazy free trial discs they sent out in the early 2000s... most people threw them away or collected them for fun. Smosh made several jokes about them:
- Much more useful were the floppies AOL (and some of its competitors) used in the 1980s and '90s… which could be erased and used for your own data. Many people never had to buy a diskette of their own even once, thanks to the mountain of them flowing through their mail slot courtesy of AOL.
- Netflix has many different avenues of giving new customers 30-day free trials.
- Skype had the seven-day free trial of group video calling, back when it was still a premium feature.
- Many antivirus software, like Kaspersky, Avast, McAfee and AVG.
- Spotify, Deezer (not available in the US), Rhapsody, TIDAL, Apple Music (for three months) and many more music streaming services.
- Scrivener by Literature & Latte.
- PowerDVD and WinDVD.