Canopus is Alpha Carinae, apparent magnitude -0.74, the second-brightest star in the night sky after Sirius, at about 310 light years, spectral type A9 II, a bright giant or supergiant, considerably more luminous than the Sun by a factor in the thousands.
Its defining feature for the Northern Hemisphere is its declination, at roughly 52 degrees south. Canopus is invisible from anywhere north of about 37 degrees north latitude, which excludes almost all of Europe, including Rome and Athens marginally, but not Alexandria, where Ptolemy worked and from which it is visible low in the south. This geographical fact is the reason Canopus appears in Ptolemy's catalogue at all and is largely absent from northern European traditions, and it is a good example of how the observer's latitude shapes what a tradition contains.
The constellation Carina is a fragment. Canopus once belonged to Argo Navis, the ship of the Argonauts, which was the largest constellation in Ptolemy's catalogue and was broken up by Lacaille in the eighteenth century into Carina the keel, Puppis the stern and Vela the sails. Canopus marks the rudder or steering oar.
Canopus has a substantial modern use: it is a standard attitude reference for spacecraft. Because it is bright, isolated and far from the ecliptic, star trackers can lock onto it unambiguously, and Canopus sensors have been standard equipment on interplanetary missions since the Mariner programme. A star used for navigation for three thousand years is still used for navigation.