There are two different process descriptions: static and dynamic.
Static Process Description:
"processContent.zip" is used in static process description. This file contains the HTML content which describe the process of this project or template. You can only access static process description from eclipse client. Open a project (assume there is a file named "processContent.zip" in the attachments list), you should see a link to the static process description in Process Description section in Overview tab. Click this link, you should see a local web page while describe the process.
Dynamic Process Description:
This is the new way to author and manage the process description which was added in 3.0. This is also the recommended way. The static process description has been deprecated. You can only access dynamic process description from Web Client. Open a project area from web client, you should see Process Description tab for this project. You can also add dynamic process description to a template which could be exported/imported together with the template.