Direct answer: Coal tar is a thick, dark liquid produced when coal is heated in the absence of air, especially during coke or coal-gas production.
Coal tar became industrially important because coke ovens and gas works produced it in large quantities. Unlike pine tar or birch tar, it is not a forest product. It comes from fossil coal and contains a complex mixture of aromatic compounds, many of which became feedstocks for dyes, chemicals, pitch, and preservatives.
Coal tar has been used in industrial chemistry, roofing, paving, wood preservation, and the production of coal-tar pitch and creosote. Historically, it also appeared in some regulated skin-treatment products, but tar.fyi/ should keep that discussion historical and regulatory rather than offering health guidance.
Coal tar is one of the main reasons the word tar needs careful handling. It can look superficially similar to wood tar, but its source, chemistry, hazards, regulation, and common uses are different. Industrial coal tar and coal-tar pitch are associated with serious occupational exposure concerns.
In short: Produced by destructive distillation or carbonisation of coal. Closely tied to coke ovens and historical gas works. Used to make coal-tar products such as pitch and creosote. Occupational exposure to coal tar and coal-tar pitch is a recognised cancer risk.