What is the difference between an etching and an engraving?

Similarities

Etching and engraving are both examples of intaglio printing. Intaglio printing is the technique in which a surface is scratched into then ink is put onto the plate and wiped away only leaving the ink in the recesses.

Identifying intaglio prints:

Plate mark: most intaglio prints will have a tell-tale plate mark, an indentation in the paper from the plate being pressed.

Raised ink levels: You may be able to physically feel and visibly see the ink above the paper, especially in the darker areas where more ink is applied.

Varying tone within a line:
Due to the varying levels of ink applied, the tone along an intaglio line or mark can vary, meaning it can become darker or lighter.

Engraving

An example of an engraving:

The Rules Banner or A compleat Ensign of the Arms of the University of Oxford and of all the Colleges therein: to which are Added those of Winchester, Sion & Westminster. also of St Pauls & Merchant Taylors Schools: Humbly Dedicated To his Grace, James, Duke of Ormond & Chancellor of the University of Oxford. By his Grace's most dutiful and Obedient Serv.t W.m Jackson.
W. Jackson ot. I.s Sympson Sculp.
[n.d., c.1730.]
Engraving. 490 x 390mm (19¼ x 15¼", with large margins. Mint.
An extremely rare architectural background with 23 roundel armorials of Colleges and schools.
Provenance: Sandys Family, Ormersley Court, Worcestershire.
[Ref: 52117] £2,300.00

Engraving is the printing process which using sharp tools to cut/scrape into a soft metal plate, typically copper. These tools are often made of steel and are called 'burins'. The engraving technique is very controlled and the lines are very purposeful with crisp edges and sharp details. To create tone in the image they lessen or increase the pressure when carving the line. This makes the line thinner in some areas and wider in others. Deep lines will hold more ink than shallow ones, which produces a darker tone when printed. Engravings can also have cross-hatched lines, sometimes filled with dots and this is called the dot and lozenge technique.

Example of the dot and lozenge technique from:

Etude dessinee par Francois Boucher premier Peintre du Roi,
Gravee par J. Edme Nochez, Dirigee par Fessard Graveur orde. du Cabinet du Roi, et de l'Academie Royale de Parme.
A Paris ches Fessard Bibliotheque du Roi, et rue Se. Anne Butte S. Roch. [n.d., c.1780.]
Engraving, Framed; 260 x 320mm. 10¼ x 12½". Some foxing, one rust spot to image.
A reclining female nude in a landscape. After François Boucher (1703 - 1770).
[Ref: 12013] £230.00 (£276.00 incl.VAT)

Etching

An example of an etching:

Long Room, Gallery of Practical Science, Adelaide Street, Strand.
Drawn & Etched by Tho.s Kearnan, 17, Euston Place, New Road [c.1830]
Etching, rare. sheet 195 x 225mm (7¾ x 8¾"). Glued to backing sheet; some foxing ; lower right corner missing; tear lower left.
The 'National Gallery of Practical Science, Blending Instruction with Amusement', which contained some 250 machines, devices and models such as a pocket thermometer, a gas mask, an oxyhydrogen microscope, a steam gun and, later, demonstrations of daguerreotypes, electricity and magnetism. The gallery was situated in the Lowther Arcade, a glass-domed arcade near the Strand in London's West End. The gallery closed in the 1840s, and the site went through various incarnations before the Lowther Arcade was demolished in 1904 to make way for Coutts' Bank.
[Ref: 34865] £140.00 (£168.00 incl.VAT)

With etching instead of laboriously cutting every groove by hand, acid is used instead. The artist coats the plate with wax (known as the ground) then uses a tool to draw into the wax; this often creates a more sketched feel to the prints. They then dip the plate into the acid and it eats away at any area that was scratched off by the artist. The longer the plate is submerged, the deeper and broader a line will become therefore making the printed line darker . By varying the length of exposure of one area over another, the etcher can change the comparative darkness. When the etcher has decided an area is dark enough they will cover that section with varnish in a process named stopping out. Another way to vary the tone of line is by making the lines in the ground earlier or later in the acid dipping process. Early lines will be darker whereas later lines will be lighter.

Links and further reading:

What is a mezzotint?

How Rembrandt Made His Etchings / Christie's

From Paper to Copper: The Engraver's Process (2009)

https://www.metmuseum.org/about-the-met/curatorial-departments/drawings-and-prints/materials-and-techniques/printmaking/etching

https://en.wikipedia.org/wiki/Intaglio_(printmaking)

https://cycleback.files.wordpress.com/2015/02/cycleback_authenticating_prints.pdf

https://voluptart.org/whats-the-difference-between-etching-and-engraving-in-art/

http://www.printsandprinciples.com/2012/11/goltzius-dotted-lozenge.html

https://blog.bookstellyouwhy.com/print-making-processes-relief-and-intaglio

https://en.wikipedia.org/wiki/History_of_printing