So from my understanding there's this, a 2003 Clone Wars, a movie in 2008, then another Clone Wars show in 2008...
But does one need to watch them all? Are they all canon? What's the difference between 2003 Clone Wars and 2008? Is 2008 a continuation of 2003?
Thanks.
2003's Clone Wars is a hand-drawn animated series by Genndy Tartakovski. It's made up of two Volumes, each one clocking at around 1 hour. It is not canon anymore. It's still a curiosity due to the animation style and the second Volume depicting the Battle of Coruscant prior to Anakin and Obi-Wan intervening in ROTS. It can most likely be found online.
It's also hard to reconcile with the films (Grievous in 2003 was just a relentless monster, extremely agile and near silent, completely at odds with how he's portrayed in ROTS) and it just doesn't work with "The Clone Wars" by Dave Filoni.
The Clone Wars movie from 2008 is more or less the pilot of the animated serie, it should be noted that it is NOT the first chronological episode and if possible, you should ideally not start with it.
The official chronological order of the series is as follow: http://www.starwars.com/news/star-wars-the-clone-wars-chronological-episodeorder (To be entirely sure: The first number indicates the season while the second and third indicate the episode number within the season.)
That should be all.
While the 2003 series isnt canon due to some small timeline discrepancies, the 2008 series pulls so many elements from it (such as Asaji Ventress) that I'd recommend watching it.
It's also the best version of General Grievous and shows how he amassed his lightsaber collection and how he was nerf'ed before Revenge of the Sith.
Plus, the animation and action sequences are still fantastic.
What Bruno said, you can really decide for yourself if the original Clone Wars cartoon is "canon", honestly nothing in it seems to contradict anything unless you wanna be really nit-picky. You get several transitions in Chapters 1-21 between Attack of the Clones and the 3D story, 3PO's gold upgrade, Anakin's Knighthood, introducing Ventress and Greivous. Then Chapters 22-25 provide a direct lead-in to Revenge of the Sith which the 3D series never quite got to, so the two stories don't really clash with each other.
Superman & The [Red-Blue] Blur = The Man of Steel