I feel marginally bad when I answer an easily Googleable question with reference to the first Wikipedia link that comes up.
I feel less bad when the easily Googleable question comes from Anon.
Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation’s Latin name. The original list of Bayer designations contained 1,564 stars.
Most of the brighter stars were assigned their first systematic names by the German astronomer Johann Bayer in 1603, in his star atlas Uranometria. Bayer assigned a lower-case Greek letter, such as alpha (α), beta (β), gamma (γ), etc., to each star he catalogued, combined with the Latin name of the star’s parent constellation in genitive (possessive) form. (See 88 modern constellations for the genitive forms.) For example, Aldebaran is designated α Tauri (pronounced Alpha Tauri), which means “Alpha of the constellation Taurus“.
A single constellation may contain fifty or more stars, but the Greek alphabet has only twenty-four letters. When these ran out, Bayer began using Latin letters: upper case A, followed by lower case bthrough z (omitting j and v), for a total of another 24 letters.
Bayer never went beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include s Carinae (s of the constellation Carina), d Centauri (d of the constellation Centaurus), G Scorpii (G of the constellation Scorpius), and N Velorum (N of the constellation Vela). The last upper-case letter used in this way was Q.