Archived projects are removed from your company's default project list and pipeline. You can view them at any time by simply clicking on ARCHIVE in the project list page. You can also access them through the search box.
To archive any project simply open the project, then in the Project Overview page, use the PROJECT STAGE indicator to change the stage to lost, completed, template, or icebox.
This is the same way you will move an archived project back to active. Simply change the stage from Lost to Estimate, Proposal, etc.