Glycaemic Index is a measure of how much/quickly the carbs in a food raise your blood sugar.
Glycaemic Load takes this a step further by taking into account the amount of carbs as well as their GI.
For some foods this works out better (melon - high GI but not that many carbs in it) than others (basmati rice - low GI but almost pure carbohydrate, so you need to have a small portion).
A stricter GL diet would have a set allowance of GLs per day; each food with carbs has a value and you'd add them up as you went.
Simplified GL plans just stipulate maximum portion sizes of low-GI carb foods and suggest not going over a certain number of portions per meal/day.