A maser was a precursor to the laser. It stands for Microwave Amplification by Stimulated Emission of Radiation, 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 waves 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.