• redshift - monotonically related to distance; linearly so with close galaxies. for stars, redshifts correspond to velocities at which stars orbit the galaxy.
• morphologies - classifications of galaxies into blobs or discs (or elliptics)
• color gradients - old stars are red, young stars are blue (burn faster). galaxies tend to have old stars in the middle and blue on the outside, giving the galaxy a 'color gradient' or perhaps more correctly an age gradient.
• stellar vs total mass - stellar is the mass we see, total mass includes dark matter. in our galaxy, 10% is stellar mass.
• Tully-Fisher relation - didn't quite catch that one.
• tau models - cheap exponential decay model for rate of star formation since the big bang. not correct, but close enough and easy to implement.