CMake target_link_libraries Interface Dependencies

If you are creating a shared library and your source cpp files #include the headers of another library (Say, QtNetwork for example), but your header files don't include QtNetwork headers, then QtNetwork is a PRIVATE dependency.

If your source files and your headers include the headers of another library, then it is a PUBLIC dependency.

If your header files other than your source files include the headers of another library, then it is an INTERFACE dependency.

Other build properties of PUBLIC and INTERFACE dependencies are propagated to consuming libraries. http://www.cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html#transitive-usage-requirements

@steveire accepted answer is great. I just wanted to add a table to quickly see the difference:

.-----------.------------------.----------------.
|           | Linked by target | Link interface |
:-----------+------------------+----------------:
| PUBLIC    |        X         |        X       |
:-----------+------------------+----------------:
| PRIVATE   |        X         |                |
:-----------+------------------+----------------:
| INTERFACE |                  |        X       |
'-----------'------------------'----------------'

• Linked by target: libraries included in target sources (not a dependency for projects linking the library).
• Link interface: libraries included in target public headers (dependencies for projects linking the library).