I guess Op wants to know the difference in order to be sure of what he's buying.
USDT is simply pegged to USD. USD is the actual currency which USDT depends on for its own price & stability. If USD rises up, USDT will likely rise up too. If USD falls, USDT will likely fall too. One follows the other... So you should always expect usdt price adjustment to tally with USD price . I don't know if this is how things are handle by Tether though.
Thank you so much for the simplest explanation. But definitely will do more research before trade it. Also, I will do more research regarding this