A Project Guide to UX Design: For user experience designers in the field or in the making, Second Edition

12. Wireframes and Annotations: Design and Direction—Before the Visual Design Begins

Wireframes and annotations are ways to identify the proposed content and structure, as well as functional behaviors, of a view of a web page or an application. When combined with site maps and task flows, these documents are also extremely useful for identifying prototyping scenarios and proofs of concept. Wireframes are typically presented in grayscale, bereft of graphical elements or finalized content; instead they use placeholder content to highlight representative locations that can be used as guidance in the visual design. Essentially, wireframes are used as a tool for communicating your designs.

Russ Unger

Basically a low-fidelity prototype of a web page ...