In the U.S., Civil War re-enacting and World War II re-enacting are considered the most popular, though other forms of re-enacting such as the Renaissance, the American Revolution, and even Vietnam War re-enacting are also in existence.
Re-enactors specialize in a form of history education known as "living history," which seeks to bring historical events mentioned in books "to life" so that those wishing to learn can get an up-close-and-personal experience.
Historically accurate clothing and accessories are a requirement to be a re-enactor, and many usually conduct their own extensive research to ensure that their historical alter egos are displayed properly.
Re-enactors are often criticized as "nerds" or "geeks" because they "dress up and run around in funny clothes and think they're cool." Also, some re-enactors are criticized because they tend to recreate military units noted as "legendary" (such as the 20th Maine in Civil War re-enacting or the 101st Airborne in WWII re-enacting) while ignoring more "average" soldiers. More simply, some people just cannot understand why re-enactors do what they do.
Still, it is worth noting that without re-enactors, the cast of historical films such as "Gettysburg" would be extremely sparse. You can't make a war movie without soldiers after all.
"Look at those re-enactors! Haha! What nerds!"
"If you didn't have re-enactors, the Pickett's Charge scene in 'Gettysburg' would have a cast of five people."