seem to be snails. depending on what type they are depends on problems down the track. if they came with the plants by mistake i dare say they are the problematic variety. these guys can breed by themselves due to haveing both the male and female genitalia.
if they are the sort im assuming. you should have an infestation in no time.
if they are unwanted as i assume they are. you can either squish them as you go or you could invest in snail eating loaches like the clown loach or pakistani loach.
but bare in mind that these loaches will inly resort to eating snails if there is no other supply of readily available foods. starve the entire tank for a few days or feed quatre of what you would normally feed and the loaches should do a good job.
you can also buy ready to use medicines to kill off the infestation but these are generally expensive considering the problem and can usualy damage you good bacteria colony.
i hope i have helped and let me know via pm if you hve any more problems.
regards, community