Good question. In unconventional machining, a form of energy is used to remove material from the workpiece. Some examples of unconventional machining processes are abrasive jet machining and water jet machining. Refer the following articles for good understanding of unconventional machining processes.
Another good “unconventional machining” process example is photo-etching / photochemical machining, a tightly controlled corrosion process which is used to produce complex metal parts with very fine detail.
Here is the wiki page and a nice video about the process