The name for a range of different instruments that can detect and quantify compounds based on their molecular weight and the number of positive or negative charges they hold (their charge state).
These instruments are often coupled with a separation technique, allowing the components of a sample to be separated and "drip fed" or eluted into the mass spectrometer to prevent overloading.
MS refers to a single experiment in which the molecular weight and charge state of a compound or compounds is detected.
MS/MS refers to a double experiment in which an MS experiment is performed, and a compound of interested selected for a second MS experiment in which structural information about the compound is obtained.
Some MS instruments are capable of accurate mass measurement, which allows the precise identification of compounds.