The main form of output on your computer is visual – using a monitor. CRT monitors used to be the standard. But now it has changed to lcd and led. When looking for monitors nowadays, you're basically looking for the same things as when you look for tv's. Of course they differ by size and resolution and vary in pricing accordingly.
Printers are used to output data in a written form or images. What to look for: Some printers can scan documents. Some print faster than others. Printing resolution also varys. Make sure to read the specs before buying one.