Seconding what Susankat said.
These are quite typical hitchhikers on live plants and they are very annoying pests that will multiply and take over your tank and clog your filters.
Snail eating fish like Yoyo loaches and clown loaches will be more than happy to keep your population down.
Or, you can try adding some aquarium salt to the water so long as you don't have scaleless fish like corydoras cats in your tank.
Still another method is to make "snail traps" of a glass bottle baited with a piece of lettuce inside. Sink it to the bottom of the trap, wait a night, and you'll have a bunch of snails in there. Then you can just pick up the trap, dispose of the snails, and do it again.
In the future, be sure to soak your plants in a solution of 1-2 tablespoons of salt in a gallon of water for a half hour to an hour. This will kill any snails, snail eggs, or bad algae on your plants.