A Muppeteer is a puppeteer for the Muppets.
Muppeteers work in several different ways to get the effect that is needed on the TV screen. Usually, Muppeteers perform their characters above their heads, standing. All of the sets are raised about 5' for this reason. To see their performance, they watch what is being taped on a monitor, or a small TV, which allows them to see what the audience will see. However, it takes a long time to get used to working in this manner, since certain things are reversed in the monitors (If a puppet turns left, it looks as if it's turning right on the monitor). Muppeteers often tape their scripts to these monitors to read off of as well.
Each show has a different way of working. On Sesame Street, for example, the street set isn't raised. Therefore, Muppeteers must use "rolling donuts" to get around a scene. These are small round seats with wheels on the bottom which let the Muppeteer move around the frame by rolling themselves around using their feet. The Muppet insert segments, however, work in the standing up position, since they're usually done on another set.
A new Muppeteer usually begins doing small roles & assisting other characters. They often right-hand, which is the term for performing the right hand of a live-hand Muppet. Muppets like Ernie, Cookie Monster & Fozzie Bear all have live-hands, which the performers wear as gloves so the puppet can pick up items (This is how Cookie Monster picks up & eats cookies.)