What is Watchmen about?

It's about who watches them. It is a complexely interwoven satire on human ideals, and heroes. Each of the main "heroes" or "Watchmen" represents a different ideology. The inherent problems of each ideology is demonstrated, indeed the problem of having an ideology at all. The relationships between each ideology is shown.

All the above is communicated with a lot of symbolism alongside storytelling, using many demonstrations of the unique properties of the graphic novel medium.

The danger of power is examined, and many times demonstrates that it is self aware as a comic, hence the pun "who watches the watchmen".