Rock candy is a type of confectionery composed of large relatively perfect crystals of sugar.
Homemade rock candy is commonly formed by allowing a solution of sugar and water to crystallize onto a string or some other surface. Superheating the solution by boiling the water before adding the sugar can help produce larger crystals. Crystals form after several days.
In the United Kingdom, there is a kind of confectionary called "seaside rock" or, more usually, "rock". This is manufactured as cylinders which have a pattern embedded throughout the length, which is often the name of the tourist (usually seaside) resort where the rock was sold. The word "candy" is rarely used for confectionary in the UK.