It's a wonderful series(wow I'm surprized that it's not very popular here) ^^
About a girl(or duck really) who turns into princess tutu(a terrific ballerina *-*) to recover the peices of Mytho(in english it's Mute for some wierd reason... anyways he's sexi :p)'s heart.
But of course, evenrything is just a story made up by Drosselmeyer(Nutcracker anyone? mhmm... they sure play a lot of Tchaicovsky) but Fakia(aka Fakir... you HATE him at the begining of the series but just wait till later -^^-) has the same ability as Drosselmeyer. Only bad thing is he's too afraid to use it because of some stuff in his past...
All the meanwhile princess Krehrue(aka Clair) steals the prince(Mytho aka Mute is called the prince by the way)'s fragment of his heart concerning love the sencond Princess Tutu(Did I mention her name was Ahiru? @.@) gives it back to him and Princess Clair(Her regular-person name is Rue btw @.@) drenches it in her father(we'll just see about that one...), the Raven,'s blood and then Mytho loves her and only her...
Poor ahiru -.-;;
Oh well.. at the end she gets the happy satisfaction of living, as a duck, with the man who truly admires her.

Wow. Rambling. I'll try to translate that post into something more readable.

A truly wonderful series, Princess Tutu(Ahiru) is about a gir--I mean duck who turns into princess tutu to recover the pieces of Mytho (Mute in English).

But of course, everything is just a story made up by Drosselmeyer. At the begining you'll hate Fakia (Fakir). Falkia holds the same power as Drosselmeyer, but is too afraid to use it for reasons known only to his self...

All being said, princess Krehrue (Clair) steals Mytho (Mute)'s heart fragment concerning love the second Princess Tutu gives it back to him and Princess Clair drenches it in her father -- the Raven -- blood so that Mytho loves her and only her.

Seems like a lot of that is spoiler material. Here's the synapse from AniDB:

Kinkan-chou is a circular shaped town surrounded by castle walls. With its calm scenery including a park, a theatre, lines of shops, it looks as if just any ordinary town... but its slightly unusual? The people who live there are all nice, but they are including cats, crocodiles, all of which can speak!

In this town, there is a large performing arts school, Kinkan-gakuen, where a girl named Ahiru learns ballet. She is a little clumsy but tries as hard as anyone else, and is always scolded and made fun of but also loved by her friends, Pike and Ririe.

Ahiru has an upperclassman, a boy whom she is fond of, Mytho. He is very good at dancing, not so talkative, but handsome; he is Ahiru`s Prince. However, Ahiru feels Mytho`s eyes look lonely. Just once, she would love to see her Prince smile! With those wishes in mind, she continues to work hard with her ballet lessons. But Mytho has a very beautiful, smart girlfriend, Rue, who is even better than her at dance. To think that someone like herself would be able to dance with Mytho is just a dream in another dream. Will Ahiru be able to become a Princess and dance with her Prince?

Seems like an interesting series, and I've never seen an anime with ballet as a somewhat central theme. I put it on my "to buy" list, because it's licensed, and I'm currently poor (likely after Christmas). More on this from me later.