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; that is, 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 here. They can also be drawn as a circle with no explicit operations detailed - right-click on the element and select the 'Use Circle Notation' option to switch between styles. Realize connectors to an Interface drawn as a circle are drawn as a solid line without target arrows.
An Interface cannot be instantiated (that is, 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, v2.1.1, p.88) states:
An interface is a kind of classifier that represents a declaration of a set of coherent public features and obligations. An interface specifies a contract; any instance of a classifier that realizes the interface must fulfill that contract. 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 instantiable. Instead, an interface specification is implemented by an instance of an instantiable classifier, which means that the instantiable classifier presents a public facade that conforms to the interface specification. Note that a given classifier may implement more than one interface and that an interface may be implemented by a number of different classifiers.