TMS stands for Transcranial Magnetic Stimulation: using noninvasive magnetic fields to safely and temporarily influence brain activity. The principle is extremely simple - an electric pulse is passed through a coil that is held against a specific location on a person's scalp. The coil is coated with isolating material, so the person doesn't get an electric shock. But an electric pulse always creates a magnetic field as well, and this field influences brain activity in the area right under the coil. Depending on the specific stimulation parameters (for example, whether pulses are repeated, and how fast), TMS can increase or decrease the excitability of the neural tissue it affects.
The direct influence on brain activity allows researchers to investigate the role that specific brain regions play in psychological functions, by examining how people's performance on certain tasks changes when TMS is applied. Using TMS therefore enables research to go beyond neuroimaging methods such as fMRI and EEG, which only give correlations: they tell us that brain activity occurs in certain brain locations at the same time that some mental process is going on, but cannot tell us whether this brain activity causes the mental process, or what specific role is plays. Directly influencing brain activity with TMS, and examining how that changes behaviour, allows for such causal inference.
TMS has been used in thousands of studies since it was first developed in 1985. Tens of thousands of volunteers have undergone TMS, and its use has led to new and important insights in a very wide range of research fields spanning all sub-fields of psychology and neuroscience, including memory, attention, perception, mathematical cognition, language, and motor function. TMS has also been used in research on a variety of neurological and psychiatric disorders, and has shown promise as a treatment tool for depression, migraines, dystonia and a number of other conditions.