The term "blacksmith", by definition, refers to someone who shapes tools, weapons and other items out of iron and steel, so called due to "black firescale", a layer of oxides that often forms on the surface of metal during heating. However, the term is often applied to those who work with metal in general nowadays, when in reality these metalworkers could be called many different things depending on the specific metal they specialized in. For example, those who specialized in bronze or copper are sometimes referred to as "redsmiths", or "coppersmiths" historically, and those that work with tin or pewter (or who finish and polish metal goods in general) are known as "whitesmiths." "Goldsmiths" and "silversmiths" are relatively self-explanatory, as they work with gold and silver alloys, respectively. In modern times, aluminum and titanium are also commonly used. In medieval England, there was another level of smith known as a "tinker", whose metalworking skills were limited to making and mending pots, pans, and other household metal items.
The craft of blacksmithing has been around since humans first learned to shape bronze alloys, though it was when iron was starting to be used that the craft truly began to take off. They were typically an invaluable member of the community until the advent of mass-production techniques in the twentieth century made their role obsolete, though in less developed areas they can often still be seen. Swordsmiths were held in high regard in Japan in particular, to the extent of attributing magical powers to their work: Masamune, it was said, made a sword that would cut everything sinful while not cutting that which was innocent. All of these are reasons the name "Smith" is so common not only in Anglophone countries but also, for example, in Slavic countries, where variations of the word "Kovač" are also one of the most common surnames, and also in Romance language with names such as Ferrari, Lefèvre, Herrera, Ferreira, and Fieraru.
The steps in smithing weapons and armor can be generally divided into forging (or "sculpting", occasionally), welding, heat-treating, and finishing. Metalworkers such as blacksmiths used a wide variety of tools in their craft, but the most common ones (and the ones you most often see associated with them) are hammers and tongs, alongside anvils to give things a steady and hard-to-move place to stand on while they work. Heating often took place on a forge that could be fueled by many things (propane, natural gas, coal, wood, etc). Some modern blacksmiths also employ blowtorches for this purpose.
While blacksmiths made many different useful items, in fiction you will generally see them specializing in weapons (especially swords), armor, fetters and chains (usually these smiths are depicted less favorably than other metalworkers) and horseshoes (a specialist in the last is often called a "farrier.") In actual history, bladesmithing and armor making were specialized professions that often had their own guilds, and the process involved a whole workshop of journeymen and apprentices rather than a lone smith at the anvil. The tools and techniques required to make arms or armor were also carefully guarded and not available to the average blacksmith or farrier, who would not be wasting his time making nails or horseshoes if he could instead be making blades or plate armor for higher profits. Blacksmiths could, in a pinch, produce rudimentary weapons and armor to equip a hastily-formed militia: this could involve modifying farm tools such as the mowing scythe and threshing flail to work better as weapons, or beating out helmets that resembled cooking pots. In much fantasy, however, an "ordinary" blacksmith may be able to produce weapons of quality well above what he would realistically be capable of.
In days of yore, smithcraft was often considered close to magic, and the smith something of a magician, because of the way he or she could turn rock into something useful. Many ancient pantheons include a god or demigod of smithing
- Hephaestus, Vulcan, Lugh, Ptah, Wayland, and many more.
Also notable is that historically, contrary to stereotypes, women were regularly involved in the trade. While it was still male-dominated, several of the disciplines of the craft, such as nails, pins, and chains, had a large following from women. They owned blacksmith shops, took apprentices, worked the forge - all of the things that mark them as “real” blacksmiths. One anecdote is from William Hutton’s History of Birmingham; he encountered a nailer’s shop in which he noted “one or more females, stripped of their upper garments, and not overcharged with the lower, wielding the hammer with all the grace of the sex.” Another common misconception is the role of muscle power. While physicality is needed to some extent, it is not quite as important as skill and technique, especially with modern tools like hydraulic presses and power drills and hammers that greatly reduce the physicality needed.
Contrary to what one might think, blacksmiths are still working today, albeit not quite in the way of ancient times. These days they tend to make specialist and custom items that are not economic for or require a level of attention to detail incompatible with mass production techniques. Such artisans will produce custom knives, film and theatre props, decorations, custom sized wrought iron gates, weapons and armour for War Reenactors, members of the Society for Creative Anachronism, LARP enthusiasts etc. and farriers are still around working for hunts, the horseracing industry, horse owners and the leisure riding industry. Other smiths produce decorative items such as candelabras.
For an index of tropes pertaining to blacksmithing, forges, metalworking, and related themes, see Smithing Tropes.