Wrought Iron is a from of Iron that is easily welded and forged. It varies from so-called 'sweet' or 'mild' iron because of it's high slag content, which is usually 1 to 3 percent per volume and is responsible for itsdark black color. It is a very low carbon metal (less than 1 percent) so, while it has a high tensile strength, it has less resistance to breakage by bending then steel.
It is said to be 'wrought' because, in most cases, the slag content is raised by adding slag to the mixture, as well as mixing and folding the metal to reduce carbon content -- the word 'wrought' means "to mould or change; to create; to elaborate".
In the early days of blacksmithing (i.e. 100 BC, really early days), the technology didn't exist to heat iron to the required temperature to separate the carbon from the iron by high-temperature smelting as we do today. Instead, early metalworkers discovered that re-melting iron a second time after the initial smelting and hammering the ingots would expel the carbon and other impurities to produce the malleable iron we now know.