UASF stands for User Activated Soft Fork.
It’s a mechanism where the activation time of a soft fork occurs on a specified date enforced by full nodes; a concept sometimes referred to as the economic majority. A UASF requires a lot of industry support and coordination. The UASF concept was combined with SegWit activation in the BIP148 proposal.
UAHF stands for User Activated Hard Fork.
Developers add a mandatory rule set to change the node software. These changes make previously invalid blocks become valid after a flag day, which does not require a majority of hash power to be enforced. Bitmain, a major mining firm, announced “A contingency plan against UASF (BIP148)” in case UASF is applied.