Projects-and-solutions Questions

Many times i have seen visual studio solutions which have multiple projects that share source files. these common source files are usually out in

I'm working on vs 2010 and myproject's type is a c# windows forms application. when i add a new project to my solution and adding references for

A friend is just starting to learn java, using intellij. he asks how can he set up some template so creating a new project will contain a default

When spliting a solution in to logical layers, when is it best to use a separate project over just grouping by a folder?