In my previous blog post, i wrote about the different deployment models available in SSIS 2012. In continuation with the previous article, the Project deployment model uses Parameters. In fact with the Project deployment model Parameters become of paramount importance. Typically in Project deployment model parameters are used to assign package properties. A project usually contains packages and parameters and is built into a project deployment file (.ispac extension). As per MSDN/Books Online
A parameter specifies the data that will be used by a package. You can scope parameters to the package level or project level with package parameters and project parameters, respectively. Parameters can be used in expressions or tasks. When the project is deployed to the catalog, you can assign a literal value for each parameter or use the default value that was assigned at design time. In place of a literal value, you can also reference an environment variable. Environment variable values are resolved at the time of package execution.
At the center of the project deployment model is the project deployment file (.ispac extension). The project deployment file is a self-contained unit of deployment that includes only the essential information about the packages and parameters in the project. The project deployment file does not capture all of the information contained in the Integration Services project file (.dtproj extension). For example, additional text files that you use for writing notes are not stored in the project deployment file and thus are not deployed to the catalog.
For more differences between Project Deployment and Package Deployment, please refer to the following link.