Generally, low throw rubbers have degrees of throw in the 30s, while 40-44 degrees of throw is roughly a medium throw rubber, and higher than 44 degrees of throw is typically considered a high throw rubber. These numbers are not exact, and may vary from manufacturer to manufacturer.
This throw angle is not an exact science (contrary to what you might think from reading on the various forums!), but more of a comparative feel between different rubbers. A rubber with a high throw is not better or worse than a rubber with a low throw, it is just different. Generally, low throw rubbers are considered to be easier to play with against topspin, while high throw rubbers are supposed to be able to lift backspin over the net more easily.