They are small black creatures similar to a very tiny slug. Try googling for a pic.
This is some more info concerning them in ponds from the BBC Nature site:
There is something off-putting about leeches, but only one species in Britain (the medicinal leech) is likely to take human blood, and it's so rare that it's unlikely to appear in your garden. There are 14 other British species, most of which are slow-moving predators of molluscs and other invertebrates. So, try to learn to love them. As for how they got there - probably in co***** attached to an aquatic plant that you brought in.