what is the difference between projects and libraries in eclipse's build path for any java project

Question

We can add project as well as a jar file in java projects build path in eclipse. so what is the use of it?

Answer 1

Projects on the build path allow you to create dependencies between projects in the same workspace as well as use existing libraries via adding jars to the same path.

For example, I may create a project which defines data types that are common between a client and server project. If I am working on all three as separate projects, both will depend on the common data types.