A FIB is a focused beam of ions, and this beam is scanned (rastered) across the sample just like it is in the SEM. The ions have energy, and when they hit the sample they create a signal from this interaction. When the energy is high enough, the FIB can remove atoms from the sample surface. The removal of material is used to cut, trench or mill the sample on a scale of nanometers to micrometers.
The following sections will describe in more detail the complex intercations between the ion beam and the sample, and will also describe the design of the instrument, and some more common applications of the FIB-SEM.