As the name suggests, the FM-T is the Migratory Tourist Form (Forma Migratoria para Turista) which is a sort of tourist visa, in the form of a fee, charged to visitors staying in the country either beyond the (tourist-) border-zone, or within the border-zone for longer than 72h. It is valid for up to 180 days. Quoting from this website:
Mexico has a tourist fee they charge visitors. If you are either staying in Mexico longer than 72 hours or traveling past the Border Zone you will need to pay the 190 peso Tourist Card fee (roughly $20 US dollars). The card, known as the Migratory Tourist Form (FMT) is valid for six months (180 days) with multiple entries. Make sure you ask for the full 180 days even is you plan to stay only a short time. You never know if you may return in those 180 days.
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According to the law you must have the Tourist Card on you while traveling (make a copy if possible in case original is lost). You can pick up a card at the border, just stop at immigration as you cross into Mexico, parking is available. You must have either a current passport or original birth certificate with current ID. Baja California and Sonora sometimes only require a current ID, but it's wise to check with the consulate first.
The FM-T is often included in the price of your airplane ticket, if you are entering Mexico by air. If not, it can be obtained at the entry point, or via your local consulate or embassy. For more information, you can also check the Visit Mexico page on the topic.
The price of an FM-T seems to be of around USD20-25 as noted on various sites, including [this Monterey Mexican consulate page and this Student Travel page. The price difference is probably due to the USD to MXN exchange rate variations.
The FM-T was substituted by the FMM in 2010. Their modality and duration seem to be the same.
