Any solution that you manually create is an unmanaged solution. You can edit, add, and create new components in unmanaged solutions.
There is an especially important concept to understand surrounding unmanaged solutions that is crucial if you wish to understand how solutions work: when you add an existing component to an unmanaged solution, the component is not copied. Instead, a reference is made in the unmanaged solution to the component in the default solution.
In the following diagram, you can see that there are several components in the Default solution. A new solution, Solution A, has been created and adds one of the existing components, Component X, to the solution. Solution A now references Component X in the Default solution:
If you create a new component in an unmanaged solution, this component is actually created in the Default solution, and in the unmanaged solution, a reference is added...