What, How, kind of Mounts

You will have the opportunity to purchase and ride a variety of mounts. You can first ride a land mount at level 20, then you will be able to ride an epic land mount at 40. Once you hit 60 you have the option of purchasing your first flying mount! (Paladins/Warlocks/Worgen have exceptions with the Ground mounts, Druids have exceptions with the Flying mounts)

Ground Mounts:

Apprentice Riding @ level 20: 4G training/1G mount (comes with "Summon <mount>" spell for Paladins/Warlocks; comes with Running Wild for Worgen)
Journeyman Riding @ level 40: 50G training/10G mount

Flying Mounts:

Expert Riding @ level 60: 250G training/50G mount (comes with "Flight Form" for Druids)
Artisan Riding @ level 70: 5000G training/100G mount
Cold Weather Flying @ 68 (to use in Northrend): 500G - can be used with either "expert" or "artisan" level mounts; trained in Borean Tundra (Valiance Keep - Alliance, Warsong Hold - Horde)
Wisdom of the Four Winds @ 90 (to use in Pandaria): 2500G - can be used with all flying mounts; trained in faction shrines within the Vale of Eternal Blossoms

Note: You have to purchase all Riding Training before proceeding to the next tier! Cold Weather Flying and Wisdom of the Four Winds are not considered a 'tier', so you can go directly from Expert Riding to CWF/WotFW.

Also note: Mounts scale with riding skill now, so if you buy a non-epic ground mount and buy Journeyman training, the mount will travel at 100% speed (up from 60% initial Apprentice tier speed). This means that Paladins/Warlocks/Druids only need to purchase the first of their mount/flight form spells if they so choose.