To get the first thing out of the way when I'm saying Clans it does not mean as in Guilds or Linkshells or Clans in that sense. In fact I'm referring to the differences in the races for the game. According to previously released information from Square Enix each race will have different clans and character creation and starting will be affected by the clan you end up picking. It seems there are two clans for each race that are selectable for character creation, whether there are more or not remains unclear but I'd guess there probably are. Looking at the new pictures that they have release seems to indicate that they are picking up a little of the tendencies from others since they have a "dark elf" type and a "dark mithra" type clan. But here from Eorzeapedia is a listing of the clans can pictures so you can get a nice visual. I'll run down all of the clans and races they are associated with here for quick reference.

Elvaan - Elezen
Forester Clan
Shader Clan

Hume - Hyur
Midlander Clan
Highlander Clan

Tarutaru - Lalafell
Plainsfolk Clan
Dunesfolk Clan

Galka - Roegadyn
Seewolf Clan
Lohengarde Clan

Mithra - Miqo'te
Seekers of the Sun Clan
Keepers of the Moon Clan

Interestingly there are no Highlander Hyur female listed. I'm guessing that is intentional from the way that they were sounding, though it is the first time that did that with anyone else other than Galka and Mithra to only have a single gender playable.

But from the pictures you can see that there are stylistic differences with the clothes as well as some appearances. Having this extra layer of detail in separation will certainly help people feel a lot more unique, which was often an issue in FFXI. It was often that you'd find yourself running around even. In the end it was the clothes that made you different.

They will be more rough and wild looking clans and more modern ones. So it'll be interesting to know if that ends up changing all clothes or it is just starting clothes. Though I'd suspect that that there will be clan specific equipment that you can get later on if FFXI is any indication. Hopefully it'll be more than just a color scheme change.

Eorzeapedia: Final Fantasy XIV (FFXIV, FF14) News, Wiki, Forums, Community and more Japanese official site update: new clan information