Intaglio is largely a process that utilizes engraving as its main method of creating images, though the use of etching, typically chemical, has been popular over the centuries to create depth and tone. A contemporary intaglio print is typically created by creating incisions on the surface of a metal plate (called drypoint, the plates are typically zinc or copper) and then etching it in chemical baths. This process is repeated and when ready, the artist will clean, ink, and wipe the plate (the ink will become embedded within the etched lines while the rest of the plate will remain ‘clean’) and then run the plate (face up) and pre soaked paper through a press.
Intaglio work is characterized by its use of fine lines, which allow for great detail, and as such has been a popular method for creating illustrations for print on currency, in newspapers, etc. Famous printmakers would include Rembrandt, William Blake, etc. It is an incredibly flexible medium. Colored inks and aquatint have been popular since at least the 1700s, though these processes have evolved considerably in the contemporary world giving way to colored stenciling (often using HIPs (high impact polystyrene) cutouts) colored gradient ink rolls, etc
Contemporary references: John O’Donnell, Magdalena (Martha) Powlowski