There are nine dungeons currently in GW2 (and some that are available during various holiday events only). You have to be a certain minimal level to enter each. Each dungeon has a story mode and an explorable mode which you can do after completing the story mode and advancing you level. It looks like you run dungeons in groups. Here's more information but once you hit level 25, the game will send you a letter explaining how you will be notified about dungeons that are available to you.
http://wiki.guildwars2.com/wiki/Dungeon
This guide to dungeons in Guild Wars 2 has a list of eight of the nine (but the guides are finished for only half of them). There's a map of their locations, but it isn't too clear.
http://guildwars2grandmaster.com/dungeon-guides/
The same website also has a guide to farming dungeons for tokens, which you trade to the dungeon's vendors.
http://guildwars2grandmaster.com/guides/farming-dungeon-tokens/
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