A yo-yo is a toy which is composed of two identical discs connected with an axle, around which a string is wound. The basic design of the yo-yo has been used to make toys for many centuries, and on several continents, as paintings and drawings from China, Europe, and Africa suggest. The modern design of the yo-yo is somewhat more streamlined, and some specially designed yo-yos have unique features, but is essentially a toy which could be made at home, with some effort. People of all ages use yo-yos around the world, although most players start when they are young.

The core of a yo-yo is the axle, which is attached to two discs, made from metal, wood, plastic, glass, or ceramic materials. The discs are equally weighted so that they will balance, and they are frequently decorated with painting or carving. Many modern yo-yos light up during use with the addition of small LEDs inside the yo-yo. A length of string is attached to the yo-yo with a slip knot on one end for the player to grasp, and a loop to go around the axle.

The loop of string around the axle contributes to the flexibility of the yo-yo as a toy. The use of a loop, rather than a knotted tie, was pioneered in the Philippines, and is known as a looped slip string. Because the string is looped, the axle of the yo-yo can rotate freely inside of it, allowing the yo-yo to spin when it reaches the end of the string, rather than just stopping. This is known as sleeping, and is the foundation of many yo-yo tricks, including the most basic trick, in which the yo-yo is brought back up the rope and into the player's hand.

To use a yo-yo, the string is wound around the axle and the loop on the free end is placed over the finger of the player. The hand is held palm down, with the yo-yo lightly grasped, and then the yo-yo is gently tossed towards the ground. The string unwinds as the yo-yo travels, until all the string has been played out and the yo-yo stops. Many players learn to do fancy yo-yo tricks, some of which are quite complicated, and strut their stuff at competitions around the world.