These icons represent what category those specific work item types are in. For instance, the clipboard with the check corresponds to a work item type that is in the task category and the one below it with the chat box and an exclamation point represents a work item type that is in the requirements category. These categories are defined in the process template when creating a team project, and they can also be exported/edited/imported to any current project. Hope this helps :)
Here is the MSDN link on customizing categories: http://msdn.microsoft.com/en-us/library/vstudio/dd273721.aspx