A maser was a precursor
to the laser
. It stands for Microwave Amplification
by Stimulated Emission
, and really, it's just a subtype
of a laser
that's easier to build. (Lasers just typically emit visible light
, or near-visible light. Masers deal with radio wave
s and microwaves. It's still electromagnetic radiation)
The first maser was built in 1953 in the US by Charles Townes and his colleagues. It was a large maze of metal tubes, with a beam of ammonia molecules shooting through it. It wasn't exactly high performace (the output was less than a microwatt) The output, however, had such a tight frequency range that it was used to run a clock very accurately.
In general, masers are used in amplifying microwave signals, with less noise added than amplification by transistors would cause. Typically, modern masers use solid-state materials such as ruby crystal.