The Installer Code is used for entering programming mode and changing system settings. (Such as adding sensors, changing delay times, changing the account number, etc.) This code is different from the master code and typically cannot be used to disarm the system.
The Master Code provides access to all daily system operations and user programming. This is the primary code used to arm and disarm the panel. If you only have one code for the system it is most likely the master code.
A User Code can perform basic arming and disarming functions. These are used for any additional persons who you want to have their own unique code to arm and disarm.
Some systems also have a Dealer Code. The dealer code is used to restrict access to the central station phone number and the download code. If your system has a dealer code programmed, the central station phone number and the download code cannot be changed unless you enter programming mode using the dealer code. All other system settings can still be accessed with the installer code.