[Matth]

A UPS saves you from dips, drops, or even complete outage - you need an appropriate power level, and to choose if you are going to protect JUST the PC (though for anything beyond "blip prevention", you would also need the monitor at least, in order to make a controlled shutdown.

Cheapest type - Offline UPS .. makes a rapid switch from filtered mains to battery power, fast enough that most things can ride the glitch.
Most expensive - Online UPS .. continuously float charges the battery while running the output through the inverter, so has no switchover, and the highest order of input to output filtering.

Intermediate - Line Interactive .. essentially an Offline type, with the additional capability to boost a present but low mains or buck a somewhat high mains voltage - while in boost/buck mode, it will typically be neither charging nor using the battery, but useful in cases where, say due to long power lines and uneven loads, the mains voltage is present but unstable, though modern active PFC power supplies with 110-240V no switch working are good at tolerating lows