Source code is often transformed by a compiler program into low-level machine code understood by the computer. The machine code might then be stored for execution at a later time. Alternatively, an interpreter can be used to analyze and perform the outcomes of the Source code program directly on the fly.
Most application softwares are distributed in a form that includes executable files, but not their Source code. If the Source code were included, it would be useful to a user, programmer, or system administrator, who may wish to modify the program or to understand how it works.
Aside from its machine-readable forms, Source code also appears in books and other media; often in the form of small code snippets, but occasionally complete codebases; a well-known case is the Source code of PGP.