A mana ability is an activated ability that creates mana (such as with Llanowar Elves, Birds of Paradise, and Ur-Golem's Eye), or a triggered ability that triggers off of mana creation and makes more mana itself (such as with Overgrowth or the last ability on Gauntlet of Power).
Mana abilities are among the few activated or triggered abilities that don't use the stack or require passing priority to resolve, so they cannot be responded to or targeted. As soon as the ability goes off, the mana is created. In practice, this simply means that a player can use mana abilities any time he or she pleases, usually while playing a spell or ability and paying the cost. Other effects of the mana ability, such as the damage that sometimes happens when using Adarkar Wastes, for example, also occur immediately and can't be responded to.
Spells that generate mana, such as Seething Song, use the stack normally, and can be responded to and targeted.
From the Comprehensive Rules: (February 1, 2009)
406. Mana Abilities
- 406.1. A mana ability is either (a) an activated ability without a target that could put mana into a player’s mana pool when it resolves or (b) a triggered ability without a target that triggers from a mana ability and could produce additional mana. A mana ability can generate other effects at the same time it produces mana.
- 406.2. Spells that put mana into a player’s mana pool aren’t mana abilities. They’re played and resolved exactly like any other spells.
- 406.3. A mana ability remains a mana ability even if the game state doesn’t allow it to produce mana.
- Example: A permanent has an ability that reads “: Add to your mana pool for each creature you control.” This is still a mana ability even if you control no creatures or if the permanent is already tapped.
- 406.4. A mana ability can be activated or triggered. Mana abilities are played and resolved like other abilities, but they don’t go on the stack, so they can’t be countered or responded to. See rule 411, “Playing Mana Abilities,” and rule 408.2, “Actions That Don’t Use the Stack.”
- 406.5. Abilities that produce mana but trigger from events other than playing mana abilities do use the stack. So do abilities that don't produce mana but trigger on playing mana abilities.
- 406.6. If a mana ability would produce one or more mana of an undefined type, it produces no mana instead.
- Example: Meteor Crater has the ability “: Choose a color of a permanent you control. Add one mana of that color to your mana pool.” If you control no colored permanents, playing Meteor Crater’s mana ability produces no mana.
- 406.7. Some abilities produce mana based on the type of mana another permanent or permanents could produce. The type of mana a permanent “could produce” at any time includes any type of mana that an ability of that permanent would generate if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability couldn’t be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.
- Example: Exotic Orchard has the ability “: Add to your mana pool one mana of any color that a land an opponent controls could produce.” If your opponent controls no lands, playing Exotic Orchard’s mana ability will produce no mana. The same is true if you and your opponent each control no lands other than Exotic Orchards. However, if you control a Forest and an Exotic Orchard, and your opponent controls an Exotic Orchard, then each Exotic Orchard could produce .
- If a mana ability can't produce any mana, it's still legal to play the ability, but no mana will be produced on resolution.
- Playing a mana ability that happens to produce no mana still counts as playing that ability. Some abilities trigger when a land is tapped for mana, and they will trigger when a land is tapped for a mana ability even if that ability generates zero mana.