FITS stands for Flexible Image Transport System. FITS
files have two parts, a header, that gives all sorts
of information including the date and time of the observation, what
object was observed, the principal investigator (PI) for the observation (the person who was granted time on the telescope to observe), and other information. The second part of a FITS file contains the actual data. The data can be in several columns (for example, time might be one column, and counts of X-ray photons per second, or rate, might be a second). The data in a FITs file can be extracted and plotted to make lightcurves, spectra or images (depending on the type of data). The columns can also be manipulated, to subtract background noise, for example, or to perform mathematical analyses on the data (more on that later!).