In very broad terms, the difference between a cameo and an intaglio is whether the gem engraving is cut into the stone or raised up from the stone.
I have always adored intaglios, the carved “into” gemstones. They are experiencing a resurgence in popularity so if you have one, bring it out!
Strictly speaking, engraving means carving in intaglio (the design is cut into the flat background of the stone). En relief (raised) carving means the design projects out of the background of the stone, exploiting the layers of a colored gemstone, which is a cameo.