Okay, I just got Nirvana, Yuna's ultimate weapon, but I don't understand what it does. Someone, help?

It's just like any other weapon; you equip it and use it to fight.

Of course, you should power it up with the Moon Crest and Moon Sigil to make it more useful.

What it does, like Jewel said, is act like a normal weapon. The only plus side to the ultimate weapons is that you can find Crests and Sigils once you have the Cloudy/Celestial Mirror to make them super-powerful without having to waste even more time trying to customize them yourself. The names of these weapons don't change either, unlike other weapons, meaning that they have special attributes. You should look it up on GameFAQs to be sure, but I think damage done by Yuna with the powered-up Nirvana is based on her MP.

Yuna's ultimate weapon Nirvana, at full power, gives her the 1 mp ability, I believe (magic abilites only cost cost 1 mp). It also gives her double experience (gain twice as much experience from enemies), triple overdrive (her overdrive guage charges 3 times as fast), and break damage limit (Yuna's attacks can do over 9,999 damage).

All ultimate weapons give extra, super powerful stats like that, but they start off extremely weak and pretty useless. To make Yuna's more powerful, you have to use the celestial mirror to activate the moon crest and moon sigil. Those will release the seals blocking the ultimate weapons true power.