Visual Studio Project vs. Solution

A solution is a container for projects, and tracks dependencies between projects.

I find some missing information in the other answers (at least for people who come from other IDEs like, say, Eclipse) . To say that a solution is a container for projects is only part of the thing. The conceptual feature of a VS project (what determines its 'granularity') is that one project produces one output: typically an executable or a library (dll). So, if you are going to code three executables that uses related code, you'll create one solution and at least three projects - probably more.