Abilities typically target another card in play, or in a players hand, deck, or discard pile
Historically, reds mana acceleration effects simply added an amount of mana to your mana pool, which you can then use to either fuel other mana accelerators to build up a Storm count, assemble a game-winning combo, or simply Fireball an opponent with lethal damage
When he is good he is unreasonably good, making 5/5 fliers for no mana or life cost, just tapping some dorks, is hard to beat
Greaves is such a popular card that two, more "balanced" versions have been printed since, in the form of Swiftfoot Boots and Lavaspur Boots, which added an equip cost and slowly eased up on the protection a bit
One curious aspect of this card is that it has a much faster infinite combo with Chain of Acid, which lets you copy infinite spells and win via the magecraft mechanic, but that can be done with any indestructible artifact
She is also a formidable combatant due to her first strike and deathtouch abilities