A feather edge is a long, narrow tool that has a tapered (feathered) edge down the length of one side. The edges are straight by design and some feather edges are advertised as having a built in straight edge, which is another name for a ruler. A feather edge is primarily used in plastering.
A feather edge is often the tool of choice for plastering because its tapered edge is narrow and can fit in corners. Its length also means it can cover a large area in one pass.
Feather edges are mainly used in smoothing. However, they can also be used to assist in other DIY jobs, such as measuring and cutting.