Again there are plenty but these are common:

Unit What it is Why use it Example
words Describe the colour in one of 140 standard colour names It's quick and easy but limits your choice of colours blue
rgb The amount of red, green and blue to mix to make almost any visible colour.  The amount can be defined in numbers from 0 to 255 or as a percentage. Very flexible and allows you to create any colour you are likely to need but it is easier if you have a tool which will provide you with the numbers you need when you click on a colour you want. rgb(255,167,56)
hex Does the same but using pairs of hexadecimal digits to define how much red, green and blue there is Exactly the same as rgb but it helps if you understand hexadecimal (which is base 16) so not as easy to use for most. #49235F
rgba Defines the three primary colours plus transparency/opacity.  The final digit is how opaque the colour should be from transparent (0) to opaque (1) Flexible and allows clever shading and transparency effects rgba(255,167,56,0.3)
hsl This is based on the use of a colour wheel.  If you understand a colour wheel then hsl is easy to understand but if not you won't!  See the next page for details but 0 is red, 120 is green and 240 is blue.  Values between those are mixes of the two adjacent primary colours.  The other values control the brightness/intensity. A more intuitive way to know what colour you will get and which colours you should use.  Like rgb but without the mystery.  Still not fully supported on all browsers in use. hsl(240, 70%, 90%)
hsla Just like rgba it adds an opacity value As above hsl(240, 70%, 90%, 0.5)

The pages under this page (those linked above) are about colour in more detail.  Look through them but if colour is not important to your Web development just use rgba and don't worry too much about it.  Those pages are for people who want their sites to look good and are willing to spend some time getting the hang of hsl and hsla.  Don't skip them completely though.