RGBA = red green blue alpha
HSL = hue saturation lightness
HSLA = the above + alpha
RGBA = extension of RGB (you know, #RRGGBB and rgb(x,y,z)) (You know how hexadecimal colours work?) It adds a fourth value to the rgb(x,y,z), making it rgba(x,y,z,o) (o is 0 to 1 inclusive, not 0 to 255)
HSL stuff = set the hue of the colour, then the saturation, then how light you want it to be. Easy as pie for beginners, but has poor legacy compat.