The carat is a standard unit of measure equal to 1/5 gram. A carat can be further sub-divided into 100 "points" allowing the easier description of stones weighing less than one carat. It should be emphasised that gem quality diamonds of over one carat are immensely rare - only one in every thousand diamonds polished will be of this size.
By far the simplest of the 4Cs, the carat is simply a standard unit of measure for the weight of a diamond. It is equal to 1/5 gram and can be further sub-divided into 100 "points".
To help give an idea of dimensions, the table below gives the approximate diameter or 'spread' of a round brilliant-cut diamond of a given weight. This should help you visualise the proportions of the diamond you are considering.
This table gives an invaluable perspective on how 'dimensional size' increases with 'carat weight'. It is worth noting for example that the diameter of a 2.00ct round diamond is only 26% greater than that of a 1.00ct round diamond.