Overalls are a type of clothing invented by a man named Levi Strauss. They are usually made of denim. Throughout much of history, many workers wore overalls as a protective clothing that keeps them from getting too dirty.
Overalls are a type of clothing like pants, except there is a bib attached and a brace on the back side. The brace is like suspenders, only they're fatter the further down you go. The brace also extends to the front, where it is refered to as straps, and is worn comfortably on the shoulders. The straps come down and connect with the bib. The straps stay connected to the bib due to gravity.
The straps have buckles on them that are placed through the buttons on each side of the bib. Gravity causes the bib to pull down, and also causes the back of the overalls to pull down too. With the back of the overalls pulling down, the force, much like a pulley, causes the straps to be pulled upwards. The combination of the force of the buckles going up keeps the bib up, and the force of the bib going down keeps the buckles and the buttons connected.
Overalls also have two or three buttons on each side of the front near the waist. They connect to the back to keep the front and back of the overalls more neatly together. The straps can be adjusted to a preferable length. On overalls for infants, usually there's only one button on each side of the waist.
Another aspect of overalls is a pocket centered on the bib. Sometimes it can be very large. Other times, it can be small, or multiple small pockets. Some pairs of overalls have a small hole on the bib pocket, where a pocket-watch can be threaded through. This hole was more practical i Overalls were a popular fashion fad in the 1990's. They lost popularity through the late 1990's. Now hardly anyone wears them. Even most manual laborers don't wear them. Whether they are unpopular for workers to wear is obviously a matter of debate. To mention which laborers were commonly associated with overalls would be considered a stereotype.
An interesting bit of trivia is that Nintendo's mascot, Mario, wears overalls. Shigeru Miyamoto, who created Mario, Donkey Kong, Kirby, and Samus from Metroid, explained the reasoning that overalls were chosen for Mario's clothing. He was called JumpMan in this game. Showing him running in Donkey Kong, an arcade game, was very difficult to animate. It was agreed that putting overalls on the pixelated character was the best way to show movement of the arms and legs.