What is an UPS? Basically, a USP (short for uninterruptible power supply), is a surge protector, a battery, and a power inverter (which turns the battery’s stored energy into usable power) combined into one system. People buy a UPS even if they have constant main power, just in case there is a blackout and they don't want to lose any data they have working on their system, or lose a moment of their favotite sports event.
Modern UPS have also started to come with laptop-style lithium batteries, and are smaller in size while providing more backup.
The best UPS for a home network: CyberPower CP685AVR (aaordable, one hour backup)
The best affordable UPS: APC BE650G1 Back-UPS (same features as top model, cheaper)
The best UPS upgrade: APC BR1000G Back-UPS Pro (more backup power)