What does it do? Allows you to pick herbs, which are primarily used for alchemy and inscription. In addition to being able to harvest herbs from the ground, you'll encounter the occasional plant-based monster that you can gather herbs from.
Do I need any gear? Though
nothing is required, carrying the Herbalist's Spade in your inventory
gives +10 to your herbalism skill. There are also bags specifically to
carry herbs if you find your inventory swamped with herbs.
Do I get any extras? Herbalists have access to buy cheap wow gold
the spell Lifeblood, an instant-cast self-heal that temporarily boosts
your haste rating. It's very useful at low levels and still provides a
small boost at higher levels.
Why take herbalism? If you want to
do Alchemy or Inscription, Herbalism is highly recommended so you can
pick your own herbs instead of having to buy them on the auction house.
But if you want to make wow money, Herbalism by itself is a good option, too -- just take your herbs to the auction house to sell.