There are many different types of belts on vehicles today. The older term for the belts we are discussing is “fan belts” or “V belts” as three or more v shaped belts moved fans etc. in the engine. Today the belts are called “serpentine belts” as they serpentine around many different pullies. A good generic term to use now is a “drive belt” which covers all the different types.
Drive belts rely on friction to do their job, so if it is slipping it will squeal. This also tells you it is not doing its job. As you will see in the pictures one belt is cracked. This tells us the rubber is old, brittle, it will brake soon and needs to be replaced. The other pic shows a new belt that will grab the pullies turning them instead of slipping. Loss of friction can be caused for many reasons and should be taken to a reputable shop to identify what is causing it to slip, yet one of the most common causes of a noise belt is that it is old, starting to crack and needs replaced.
Drive belts need to be replaced before they brake as they can cause more damage to hoses and radiators when they whip around the engine. Plus, a broken or slipping belt can cause the engine to overheat or leave you stranded on the side of the road.