WoWDB

These tickets are rewarded for doing quests at the Darkmoon Faire.  Some quests can be completed once per Faire/Month, while some are dailies and can be done every day.  The full list of quests that rewards these tickets is here.

They can be used to purchase a number of things: pets, mounts, heirlooms and miscellaneous fun items.  Please see the full list here.

You can earn up to 145 of these per Faire, assuming you do all the monthly quests (including turn-ins) and the dailies every day the Faire is active.