A wireframe is a design tool used to easily communicate ideas, and allow for quick iteration. Wireframes can be created easily by anyone with or without technical know-how to discuss ideas.
Often when I work on projects with CEO's and high-level executives there is a problem communicating design, concepts and intent. People may cite the lack of design or drawing skills. A wireframe levels the playing field allowing anyone with even basic Powerpoint skills to create basic screen designs. I posted my original wireframe for web-applications here. I'm now back with an iPhone based version.
Why iPhone? Well first off I've been doing a number of iPhone related projects including Runkeeper and GPSTwit among others. Secondly the iPhone provides a unique interface language and set of reusable design patterns that can be easily incorporated into new applications. While the basic wireframe components can be used to conceptualize any mobile device the iPhone interface makes it uniquly different.
The primary design pattern used on the phone is the list pattern. There are many examples of the list pattern but the basic idea is that it allows you to add/edit/remove/view sets of items. The second design pattern is the table pattern usually used for forms, input and settings. Combined these two elements form the foundation of the platform.
The wireframe provides a number of examples of how these patterns can be used, modified or altered.