An Interface is a specification of behavior (or contract) that implementers agree to meet. By implementing an Interface, Classes are guaranteed to support a required behavior, which enables the system to treat non-related elements in the same way; ie. through the common interface. You also use Interfaces in a Composite Structure diagram.
Interfaces are drawn in a similar way to a Class, with operations specified, as shown below. They can also be drawn as a circle with no explicit operations detailed. Use the right-click context menu option Use Circle Notation to switch between styles. Realization links to an Interface drawn as a circle are drawn without target arrows.
Note: An Interface cannot be instantiated (ie. you cannot create an object from an Interface). You must create a Class that 'implements' the Interface specification, and in the Class body place operations for each of the Interface operations. You can then instantiate the Class.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 114) states:
"An interface is a kind of classifier that represents a declaration of a set of coherent public features and obligations. In a sense, an interface specifies a kind of contract which must be fulfilled by any instance of a classifier that realizes the interface. The obligations that may be associated with an interface are in the form of various kinds of constraints (such as pre- and post-conditions) or protocol specifications, which may impose ordering restrictions on interactions through the interface. Since interfaces are declarations, they are not directly instantiable. Instead, an interface specification is realized by an instance of a classifier, such as a class, which means that it presents a public facade that conforms to the interface specification. Note that a given classifier may realize more than one interface and that an interface may be realized by a number of different classifiers."