Vienna has 23 districts in total.
The first district is where you will likely spend most of your time. This was where the entire city of Vienna was located until the middle of the 19th century, until the city walls were removed to create the Ringstrasse, which now encircles the old city.
The first district is surrounded by districts two to nine, which are known as the inner districts (Innenbezirke). These districts are also surrounded by a famous street, called the Gürtel, where Vienna's red light district can be found.
Outside the Gürtel, you can find Vienna's other 14 districts.
By looking at an address, you can see where a particular point of interest is by inspecting the postcode. All Vienna addresses start with a 1. The next two numbers designate the district. Most attractions have therefore the postcode 1010, which designates the first district.