The first 1 is lvl 30, and you get access to more as you level.
Ok, so each mob and stuff in the dungeons has a chance to drop a 'shard'. Shards are basicly dungeon currency exclusive to that particular dungeon. Dungeon x will give 'x shards' and y will give 'y shards' for example, and there is a vendor for each dungeon at the fort in Lions Arch. You spend the dungeon currency at the vendor for weapons/armour potions that help do that dungeon. The stats vary a lot in usefulness (there are only 3 lvl80 dungeons right now, and ofcourse the lowest lvl dungeon stuff is useless) BUT the weapons and armour LOOK unique.
Also, if you go to Heart of the Mists, on your immediate right there is a 'pvp locker' you can go here and look at all the armour and weapon skins you get from the dungeons (maybe more), by r-clicking and pressing preview. You can preview what all the high end armour looks like, as well as the low end if you like the look, then you can know what you need/want to farm.
So, it's ok and even preferable to run the lvl80dungeons to get good stat gear, then run the lower end dungeons to get the 'look' you want for said armour by using transmutation stones. I.E. if you don't like the look of the lava dungeon with big armour and flames etc, but like the stats, you can farm it, then run for example, the nighmare court dungeon that gives stuff to make you look like the Nightmare Court people.
Hope this helps, and maybe sheds a bit of light on the dungeons. Not exactly sure HOW you get the shards, but I'm just guessing they drop of mobs etc.