the postcard from animation mentor

what's meaning of CA in 'OAKLAND CA'?

today, i get a postcard from animation mentor online school. cause my english is poor, there's a question about this postcard. so i ask Bill by skype. i got the meaning later,and i know some information about american.

here is the dialog between i and Bill(if i forget it ,this blog can tell me! 猫咪胡须 is my nickname in china.) :

猫咪胡须: 15:26:17
OAKLAND CA 中，CA代表什么？加拿大？可是OAKLAND是美国的地名啊！
bill: 15:28:23
Sorry I am too busy to talk to you. But I would write.
bill: 15:28:37
CA means California.
猫咪胡须: 15:29:02
i know! thanks!
bill: 15:29:13
OK。
猫咪胡须: 15:30:51
so, Oakland is a city in California?
猫咪胡须: 15:31:29
so, Oakland is a city in California provice?
bill: 15:32:35
Yes, a city in North California. California is called a State not Province. Oakland is about 20 miles south to Berkeley.
猫咪胡须: 15:33:27
THANKS AGAIN!
bill: 15:34:35
No Problem. Berkeley is a city in East San Francisco Bay. Oposite to Berkeley is San Francisco.
猫咪胡须: 15:40:20
i signed up my information in a website about animation mentor school.today i get a postcard from that school.the question about CA just from the stamp on the postcard.
bill: 15:41:06
Great.
猫咪胡须: 15:46:26
so ,go on work please! i'm learning knowledge about animation and 3d animation.have a good mid-autumn-festival !
猫咪胡须: 15:46:30
bye!
bill: 15:47:34
Thank you. Good luck, and have a nice National Holidays.