Wireframes are essentially professional mockups of a product to give a sense of how it looks and feels without having to actually develop it. One of the first steps for a startup is to create a mockup of the layout and design of the app, and what the navigation and workflow is like. Instead of trying to draw with pen & paper, balsamiq lets you drag-and-drop resizable sketches of common website/app elements like an iPhone or browser window, buttons, icons, etc. No coding or artistic abilities required!

Also very useful later down the road when you want to make website revamps and need to see how it would look before investing the time to actually change it.