High Elfs have the best magicka (+50) and the ability to regenerate magicka* at a faster speed but they're the most vulnerable race in the entire game. Bretons don't really have an advantages (in skyrim) for casting spells but they're much better at taking them and can in bad situations convert half the power of incoming spells into magicka*.
*once per day
*once per day
How are Altmers the most vulnerable race in game? In Skyrim they no longer have the weaknesses they hat in previous TES games and in Skyrim they have the highest speed and Damage multiplier of all races due to their size? There was a thread here on the forums that showed that (Can not find it at the moment, maybe some else can.) and the UESP Wiki shows this information as well. (Scroll down to the Notes part.)
http://www.uesp.net/wiki/Skyrim:Altmer
EDIT: Ah i found the thread.

http://www.gamesas.com/index.php?/topic/1286166-hidden-stat-differences-based-on-race-selection/