A femtosecond is a million times shorter than a nanosecond. Yes, that's very fast. It's "ultrafast".

In the mathematician's lexicon, a femtosecond is 1x10^-15 seconds.
In words, it's a quadrillionth of a second.

If one assumes the universe is 12 billion years old, a femtosecond compares to a second as 10 minutes compares to the life of the universe.

A femtosecond is one millionth part of one billionth of a second. This is more than a million times shorter than the several nanosecond duration pulses used in more traditional standard industrial micromachining systems. There are many advantages to micromachining with these much shorter pulses as described on the Time Scales page.

The lasers typically used in most industrial machining applications have pulse widths on the order of several billionths of a second (several nanoseconds) or longer. These "long pulse" sources work by first melting the material and then vaporizing it. The process is analogous to a volcano, with material splattered onto the surrounding surface and slag formed around the hole.

In femtosecond or ultrafast laser pulse micromachining there is no melt phase. The material undergoes a transition directly into the vapor phase. This is because the light pulse has a duration that is so short that there isn’t enough time for heat to propagate into the surrounding material. The femtosecond light pulse creates a solid density plasma (a mixture of loosely bound ions and electrons). This plasma expands away from the material in a very highly ionized state, taking most of the energy away with it. Consequently, very little energy is left behind to create an undesirable melt that can solidify into a slag.