What does it do? Allows you to mine for ore and gems, which are used for blacksmithing, engineering, and jewelcrafting. In addition to being able to mine ore nodes, you'll encounter the occasional earth-based monster that you can mine for ore after death.
Do I need any gear? Though you don't need any special tools to mine, carrying a Mining Pick in your bag will give you +10 to mining skill. If you carry lots of ore or us wow gold gems, you may also want to invest in a mining bag.
Do I get any extras? Miners have access to the passive ability Toughness, which gives a bonus to stamina. It's a decent buff for any player, though tanks will certainly be appreciative of the extra health. If you want to pick up blacksmithing, engineering, or jewelcrafting, mining is highly recommended as a companion profession. However, just like herbalism, mining is good on its own as a money-maker, too.
Why take mining? Mining is a must if you plan to pick up blacksmithing, engineering, or jewelcrafting -- but by the same token, it's a great money-maker by itself. Because the materials gathered by selling wow accounts mining are used in more professions than any other, they're typically in high demand on the auction house: with some savvy trading skills, your character could make a bundle.