A solution package is the distribution package (file extension .wsp) that delivers your custom SharePoint code and artefacts to the web servers within your server farm.
Components that can be distributed in a solution package include:
- .NET Framework assemblies that contain the code that drive the solution.
- Deployment files such as resource files, images, or other helper files.
- Many solutions involve the delivery of new templates and definitions for sites, lists, libraries, fields, content types, and more. These definitions are in the form of CAML-based XML files.
- Configurations that must be performed at the Web-server level — for example to the web.config files for the registration of Web Parts.
Reference (2012-03-17): http://technet.microsoft.com/en-us/library/cc263289(v=office.12).aspx