By linking a contact list to a Twitter.com account you have the option to automatically submit a post to Twitter containing the title of the campaign and a link to view it.  When people click that link and view the campaign their actions (such as clicking links) will be tracked and shown in your campaign reports. 

To setup the connection, click on Lists > Manage Lists.  Click "Edit" for the list that you want to link to Twitter and then click the "External" tab. You can then connect the list to your twitter account.

You will always have the option to choose whether or not to post a campaign to Twitter when sending each campaign.