i usually have to build on top of APIs or other projects. i am not very familiar with eclipse, so what would be the difference between import project and create project on existing source
As far as I know, Import Project is for projects that you have created either using Eclipse or other compatible IDEs, and that have a project file associated to them.
Meanwhile, create project from existing source, is meant for creating a new project (creating a new project file) when you only have the source code for the project and no associated project files.
While the first option will attempt to import settings such as the runtime you want it to run on, etc. the second will simply go with the default eclipse options.