A melon baller is device for cutting nice, even, spherical pieces of melon, for use in salads, cocktails etc.
Most melon ballers have a thick handle, a thin neck, and then a fine, sharp, hemispherical metal scoop at the end, usually with a hole in the back to allow the passage of juice and air.
Most melon ballers will give you a ball about an inch in diameter, although many will have a scoop at both ends, each being a different size. Vive le difference!
For the best texture to your melon balls, chill the baller before use. This helps prevent the 'slimy' texture that some melons can get when cut.