More than 16 million people call Cairo home and it's chaotic, exotic, smelly, dusty and also beautiful. Perhaps the most interesting section of Cairo is Medieval (Islamic) Cairo
. Medieval Cairo is a warren of streets just bustling with life. There are mosques at every corner, Coptic churches, huge medieval gates and bazaars selling everything from motorbike parts to perfumes. Highlights include the Citadel and the Khan Al-Khalili bazaar (for all your tourist trinkets
). The major mosques worth visiting include: the Mosque of Mohammed Ali; the Ibn Tulun, one of the largest mosques in the world; and the Al-Azharmosque which houses the oldest university in the world (from 970AD).
The New York Times has a nice article describing Medieval Cairo in all its glory (and squalor). See my Cairo city guide for more information about visiting Cairo.