The series follows Sakura and Syaoran as they search for Sakura's memories which have been scattered across numerous parallel worlds in the form of feathers. Joining them on their journey are the warrior Kurogane, the mage Fai, or Fye, together with magical ball of fluff Mokona.
Tsubasa Reservoir Chronicle's is highly enjoyable anime series that has some nice characters and an interesting plot. Regarding the English dub it is relatively good and the soundtrack is beautiful. The series does have a brief crossover with `xxxHolic', which was nice, and it is just a sad that the `xxxHolic' anime didn't follow suit.
Clamp fans will recognize Syaoran and Sakura from Cardcaptors (Disc three has the story of their childhood past) and indeed some might suggest this is a more shonen take on that story but whilst there is some truth in it this goes deeper into the characters.