To save an article as work in progress, use either the "Save this article as work in progress" or "Save this article as work in progress and continue" option in the action panel. Both will save the article's details without submitting it for approval or to the live site.
If you save another user's live article as work in progress, the original article will effectively be "locked" to you - it won't be possible to edit the existing article while the work in progress version exists. If you then later submit the article, you will take over responsibility for the changes.
If you save one of your own "awaiting approval" articles as work in progress, then the approval process will be suspended.
If you have saved articles as work in progress, iCM will include a "Work in Progress Articles" entry within the "Your tasks" list on both the iCM and the articles home page.
Double-click on this task within the list to navigate to a list of your work in progress articles. iCM will also include a "My work in progress articles" option within the actions panel of the articles home page.