Transit spectroscopy is a general term for the use of an extra-solar planet's transit to carry out transmission spectroscopy on the planet's atmosphere. It is also differential spectroscopy, using the differences between the spectrum during and outside the transit to isolate the characteristics due to the planet and its atmosphere. Because the atmosphere transmits some wavelengths and absorbs others, the reduction in electromagnetic radiation (EMR) from the star varies by wavelength, revealing information about the planet's atmosphere.
Transit spectroscopy of host stars' Lyman alpha emission lines have been used to detect and study hydrogen exospheres of exoplanets and their atmospheric escape.
Occultation spectroscopy is a similar term for the occultation (i.e., secondary eclipse) of a planet and analysis of the difference as well as the light curves at different wavelengths at the beginning and end of the eclipse reveal information about EMR emitted (the planet emission) and reflected (due to albedo) by the planet.