Teeny critters in my tank, what are they?


hi, im a newbie to this hobby and i was just lookin at one of my snails crawling on the sand right now, and i noticed tiny little brown things smaller the size of a period (.) crawling around my sand and glass. when i looked all over the substrate, i noticed hundreds more of these tiny critters crawlin around, and boy do they move fast! i even saw some jump. are they plankton or some kind of animals that gro from the LR?