The snail that usually hitch a ride with plants is a common pest snail, the type that multiplies to no end and is usually only brought under control with a loach in the tank. Mind you they certainly do come handy in removing excess food and also aid in hatching fry, as they eat the dead egg shell.
The Malaysian trumpet snail rarely get to your tank that easily, as it lives in the sand, and only comes out if there's food available, and at night time.
The snail that hatch a ride with plants are round and black, the Malyasian trumpet snail has an elongated pointy shell, and is a brown/beige color.
Like Dr. P says, do a search on google and you'll find pictures and more information about them
