Wireframing is the next process on from prototyping. Once a design has been created, it is important to communicate the design so that everyone can see how it should work.
Simple images aren't enough to convey this, wireframes include notes on content, functionality, behaviour and supported controls. This is particularly important for responsive interfaces and designs supported across tablets/phones.
We can create wireframes in a number of styles, from simple images with callouts to full specifications of behaviour. The depth we document to really depends on your need.