Betelgeuse is Alpha Orionis, a red supergiant of spectral type M1-M2 Ia-ab, at roughly 640 light years, with an apparent magnitude that varies irregularly around 0.5.
It is enormous and its size is genuinely uncertain: estimates of its radius range widely, but it is of the order of several hundred to a thousand times the Sun's, large enough to swallow the orbit of Mars and possibly Jupiter if placed at the centre of the solar system. Its outline is not sharp, the star has no well-defined surface, being surrounded by an extended, irregular, convecting atmosphere with enormous convection cells. It was the first star other than the Sun whose disc was resolved, by Michelson and Pease in 1920 using an interferometer, and the first to be imaged with visible surface detail.
In late 2019 and early 2020 Betelgeuse dimmed dramatically, dropping to around magnitude 1.6, its faintest in a century of records, and prompting widespread speculation that it was about to explode. It recovered. The explanation now generally accepted is a dust cloud: the star ejected material which condensed into obscuring dust in front of it, combined with a cool patch on the surface.
Betelgeuse will end as a supernova, and when it does it will be bright enough to be visible in daylight. The timescale is of order a hundred thousand years, which is imminent for a star and unhelpfully long for anyone waiting.
Despite the Alpha designation it is usually fainter than Rigel, Beta Orionis, Bayer's letters do not strictly follow brightness, and Betelgeuse's variability may have contributed.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy