Problem
A small label, descriptor or additional piece of information is required for a piece of page content, a component or a control.
Solution
Tooltips are transient, usually contextual, informational assistance widgets. They are initiated by the user hovering over a potentially interesting target, or automatically presented when the system determines the user needs help -- such as first time, or a change in the system since last visit (put these notes over there when done here).
Information presented in a tooltip is a helpful label or content add on. It is not content itself, but serves to clarify short labels or icons, or explain jargon, requirements or systematic needs that may not be clear.
When a function like a tooltip is called for, but it may be deliberately displayed by the user, or has interactivity in the information label itself, this is instead an Annotation. See that pattern for more details and comparison.
Variations
hover adjacent
fixed location, usually docked to the top or bottom of the viewport
Interaction Details
There is usually no explicit method to present tooltips. For any systems which support hover states (mostly scroll-and-select), any item in focus may display a Tooltip label. For any device, including touch and pen devices without a hover paradigm, tooltips can be used by the system to point out unused tools, or new features. The lack of contextuality makes this less immediately useful, and the items to be highlighted must be carefully selected to avoid over-selling a single new feature.
Certain desktop systems allow entering a "help mode" where tooltips appear instantly (though still on hover), for all page items, but this has not yet become common in mobile so cannot be considered a pattern yet.
The floating information labels may not be selected in any way. They should be built so they do not exist as far as selection mechanisms go. For example, if a tooltip is partly obscuring a function in a touch or pen device, if the tooltip label is under the contact point it should be ignored, and the item behind it activated as though it is visible.
When the label is being displayed, it has to get out of the way of other actions the user might request. If any selection is made elsewhere on the page, including entering text input fields, it should be disappear immediately.
Presentation Details
Tooltip labels should be presented after a brief pause...
disappear after a brief delay, unless another appears first... never display two at once.
The label must be clearly not part of the page context. Use borders, shadows, and transitions to make this clear. It is often best to make it appear as though the Tooltip label is floating above the image, or protruding from the page. Certain practices described in the Simulated 3D Effects pattern may be used to emphasize this.
Antipatterns
Do not rely on tooltips to solve interaction and interface design problems. If icons are unclear, make better ones, or apply fixed labels, for example.
Labels should never exceed the space available. Do not let them simply float off the page. Label text should not end in ellipsis or wrap to a second line. Multiple lines of information may be displayed, but each line should carry it's own information.