After installing Google Plugin for Eclipse, there are options to create three different types of projects
What is the difference between these three?
App Engine allows you to deploy application with minimum infrastructure related configuration. Standard is quicker in both deploying and scaling your application, but has a limited choice of languages and libraries to use. Flexible manages docker containers so it can run almost anything and has more powerful machines to chose from, same that Compute Engine has, but it takes longer to deploy and is in general more expensive.
Lightweight generic code that should be always available should run on Standard. A batch job that is processed once every day and requires an expensive setup and third party libraries would better run on Flex.
More on that here
Google App Engine Dataflow Java Project
It's not an App Engine, it says:
Google App Engine Dataflow Java Project
It's a service directed specifically at batch jobs and streaming. More on it here.
