They're two different verbs;
to be sitting:
å sitte - sitter - satt - har sittet
De satt på gulvet - They sat on the floor
This means that they were already seated, and is just describing that they're currently sitting. Note that this verb doesn't take any objects.
å sette - setter - satte - har satt
Jeg satte den på bordet - I put it on the table
Du setter deg på gulvet - You're sitting down on the floor
This means that you were standing, and are now in the act of sitting down. Note that this verb always takes an object.