They belong to the protists and mainly live in sea and fresh water. They are surrounded by so-called flagella, which have a similar function to cilia, but a different structure.
Explanation for children:
Dinoflagellates belong to the protists and live in the water. They are called like this because they have flagella. They are used for locomotion, but are not as numerous as cilia. They can be compared to little whips, the lash of the whip causes movement.