This is the Nasir al-Mulk Mosque in Shiraz, Iran, and must be one of the most striking mosques because of it's amazing color, or what the Huffington Post calls, its "riotous wonderland of color." The Post has more amazing photographs that you should see.
Islamic art shies away from figural representation. In fact, religious leaders forbid the depiction of human and animal figures. Consequently, calligraphy and geometric patterns became popular. You can see the geometric patterns in the features of the mosque above.