Application developpers like to illustrate the process and layout of the work they are planning. Customers like to have a visual sample of what's coming, and that's exactly the kind of step that clarifies discussions when negociating work and contracts. Balsamiq mockups is a basic ilsutration application that you can use to very quickly build a UI mockup. A lot of softwares do that, wbut this one is rellay simple to use and very light to install, the free version is actually browser based ! Mockup below took approximately... 15 minutes !