Derbent claims to be the oldest city in Russia, and on the postcard (above) you see the old Armenian church and in the background, the Sassanid Fortress. Its population is part of a variety of different ethnicities and religions.
Dagestan is in the Caucasus, and borders Chechnya (another Russian republic) to the West, the Caspian Sea to the East, Azerbaijan to the South and Georgia to the South West. Most of its population (around 80%) is Muslim. Dagestan used to be a part of different Persian kingdoms, before coming under Russian control.
Dagestan was part of the briefly independent Mountainous Republic of the Northern Caucasus, that proclaimed its separation from Russia during the February Revolution of 1917, but was captured by Soviet troops in 1920. Below you see the current flag of the Republic of Dagestan.
Here are some more pictures from Dagestan, starting with two from the beautiful colour photographs taken by Sergey Prokudin-Gorsky in the first decades of the 20th Century.
More images, this time from Flickr.
Un bolshakov on Flickr.
And some videos about the culture of Dagestani people.