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  The next step in creating an executable program is to process the source
  code with a language translator. For assembly language, the translator is
  called an assembler; for high-level languages like Pascal and C, the
  translator is called a compiler. A translator converts source code into
  machine-language instructions, in a form known as object code. Object code
  contains executable machine code, but also includes additional information
  about the structure of the executable program. The object-code format is
  designed so that separate object modules can be combined into a single,
  unified program. Object-code files, by convention, have a filename
  extension of .OBJ.

  You can also use an interpreter to translate a program built from separate
  source-code modules. Interpreters, however, are rarely capable of
  generating object code, so binding separate program modules together
  generally relies on improvised language-specific programming, as we'll see
  in Chapter 20.