A wireframe is a visual guide that represents the skeletal framework of a website. It is also known as a page schematic or screen blueprint. These wireframes  provide low-priced and highly effective support for iterative design. The best thing about them is that they are extremely easy to create and change, where as changing a a coded web app is very tough. Wireframe of a web app displays a conceptual screen design and task flow.
You can create and test them. Because of their flexibility, they can prove to be really good when it comes to staying away from poorly conceived, ineffective designs.

At the beginning, your wireframes can be basic sketches on paper or a whiteboard, to help conceptualizing the user interface and task flow. These kinds of wire frames can be good enough for small projects, and you can start the development process. But for most of the projects, you would have to move from sketches to diagrams made with the help of some software like Adobe Illustrator, Visio, or OmniGraffle. These wireframes are really good for big projects as they can be changed easily and can also be distributed among the design team with ease.