What are SMA connectors and what are they used for?
This is an example of SMA plug and jack connectors:



Most often, the plug type (left) is used on cables and the jack type (right) is used on instrument panels or pcbs.

SMA connectors are:


Small relative to connectors that preceded them, like BNC and N types.
Suitable for use at frequencies up to 18 GHz.
Widely used in industry for those frequencies.
Quite robust against vibration, repeated insertions, and other abuse (when a proper torque wrenche is used)
Available at reasonable prices (e.g. on the order of $5 per connector)
Available on a wide range of coaxial cable types.