HSL (Hue, Saturation, Luminosity) allows us to describe meaningful relationships between colors. Give this brown color, hsl(36, 73%, 10%), it's clear that if we desaturate 40 steps and lighten 70 steps we get hsl(36, 33%, 80%), a cream color. Look at that in hex, #2C1D07 to #DDCFBB, or in rgb, rgb(44, 29, 7) to rgb(221, 207, 187), and the relationship between colors isn't evident in any meaningful way.
I use Compass and Sass for all of my web design work and there are great color functions that allow me to manipulate colors using these relationships. I can use Sass's adjust-color to convert brown to cream like in the example above.
adjust-color(#2C1D07, $saturation: -40%, $lightness: 70%)
//returns #DDCFBB
Sass converts easily between rgb, hsl, and hex so I can have convenience of HSL color relationships but the browser compatibility of hex color notation. There are other color functions like change-color which lets you set the properties of a color, and scale-color which applies color transformations on a relative scale. These are great, but because they work like HSL, you have to understand how HSL works.
In graphics software I pick colors in HSB (Hue, Saturation, Brightness) because it feels more natural to work with than RGB or CMYK. Now, with CSS3 we can use HSL which is actually quite different than HSB. Without a decent HSL color picker, it's difficult to understand.
Pick a Hue from 0 to 360 and with saturation at 100 and lightness at 50 and you'll have the purest form of that color. Reduce the saturation and you move towards gray. Increasing the brightness moves you towards white, decreasing it moves you towards black.