As its name suggests, dual link has two physical digital data transmission channels where single link only has one. The practical outcome of this is that with single link you are limited to a data bandwidth of just over 4gb/s. This means that on single link you have a maximum colour depth, refresh rate and resolution displayable (the higher the resolution you chose, the lower the colour depth and refresh rate that can be displayed etc.) For most applications this means 1920x1200 at normal refresh and colour depth.
Obviously, for larger monitors you will need dual link to adequately feed the data to run them.