Non-Gamer's Guide to This Post
Mount: At level 40 you can train (90g) to ride a mount (10g), which will increase your movement speed by 60%. At level 60 you can train (900g) to ride an epic mount (100g), which will increase your movement speed by 100%.
If you have the expansion pack, you can get a flying mount (5,000g) that can only be used in the Outlands.
You can't tell me that's not the most precious thing you've ever seen. Seriously. I would get every last one of my characters a cow if I could.
You can have more than one mount -- as many as you can hold, I guess. Each race has its own mount, but you can get another race's mount if you achieve Exalted status with that race (by doing quests and earning reputation).
My human is going to get a ram mount . . . eventually. After I'm allowed to play her again (she's my grouping character, but we're grouping on horde), and after I reach high enough level to work for dwarven reputation. The rams are just so CUTE! ^_^ A-do-ra-ble!
There is one really tall, fancy ram that you can get from playing battlegrounds. I prefer the squat, happy dwarven rams, but this is an interesting alternative.
Fancy Ram -------------->
There are other PVP mounts available from playing the battlegrounds, but the ram that is available is black. I don't know how my holy priest would look on black, but maybe it'd be worth it?
Any other info you want on mounts is here.