Originally patented by A.C. Gilbert (today owned by Meccano), an Erector Set is a brand of construction sets from metal. It consists of pulleys, shaft collars and axles, plates and girderswheels, gears and electric motors and various metal strips with holes and nuts and bolts to connect the pieces.
The first set was manufactured in 1913 and was called "The Erector / Structural Steel and Electro-Mechanical Builder". The name was changed to "The Mysto Erector", "The Toy That Resembles Structural Steel" in 1914. Erector quickly became the most popular toy construction set in the US, because you could take it apart and build something entirely different with it. And it was the only construction toy set to contain a motor.
What's it worth? Take a look at this Erector Set price guide: sold listings for a value indication.