When building for a particular product, it's often useful to have minor variations on what is ultimately the final release build. By using different build variants, it can help the different parties in the product development cycle. There are primarily three kinds of build variant in the AOSP so far. The engineering build is the default one and is suitable for development work. In this type of build, the product security policy is not fully enforced and the debugging mechanisms are turned on. It is easy for engineers to test and fix issues with an engineering build.
The second flavor is user build, which is used for the final release. All debugging mechanisms are turned off and the product security policy is fully enforced. The third flavor is userdebug, which is in between the engineering build and user build. This type of build can be used in the field test, which is also used by the end users...