Different doors provide different protection and resistance. Depending on what country you live in or where you buy the door from the rating is given by an appropriate regulating and testing company. USA and EU have their own fire test standards, which must be met by certified manufacturers. This certification is the direct indication of what you can expect from the specific door.
So when you are looking at a particular model always check what rating it has and who certified it. For example in USA you will most likely see the rating approved by UL company. If you would like to know more about UL you can read about them in this post
The rating for fireproof door is given in minutes – 30, 60, 90, 120. As you probably have already guessed the number of minutes means how long it can resist fire. So if you live in EU or UK you will see labels like FD30, FD60, FD90 etc. If you live in USA you will see a fire rating label like – 0.5 hour, 1 hour, 2 hour etc.
Heavy duty steel doors normally provide higher fire rating – up to 4 hours. Timber doors bear lower ratings: usually 20-60 minutes.
Important note: The door itself and its connected parts (an intumescent strip for example) have independent ratings as they are made of different materials.