Tooltips offer UI designers the reassurance that even if there is some ambiguity as to the function of a link, label, button or call to action we can sleep easy in the knowledge that we can add a tooltip with additional information to reassure our users that they are making the correct choice. Tooltips should not be used as a primary source or information but more as a secondary source to supplement icons and labels etc to help our users better navigate a section of our design.
Tooltips in mobile and touch interfaces can sometimes be accessed by the first tap displaying the tooltip and the second actually executing the action however this can be cumbersome and is not always preferable. Another way to add this additional information to a mobile device is through the use of coach marks. A nice overview can be seen here on pttrns.com. While coach marks should be used selectively they are a great way to either initially introduce a user to an interface features or to demonstrate that there is additional functionality available.