The Bayer designation of a star is a common type of star designator consisting of the constellation name in Latin genitive form prefixed by a letter from the Greek alphabet. For example, "α Orionis" represents some particular star in the constellation Orion. The designation system was developed by astronomer Johann Bayer for his 1603 astronomical catalog, Uranometria. Bayer's assigned designations are still used, and for some stars, it is the usual designator.
Bayer generally labeled the stars in the order of apparent magnitude, the brightest being α, the next, β, but the designations do not always follow this exactly: an exact ordering would imply that the brightest star in Orion would be "α Orionis", but this designation is given to Betelgeuse, a bright star in Orion, but Rigel (designated "β Orionis") is brighter. ε Indi is an example of a Bayer designation still in use.
To include additional stars in a constellation beyond the number in the Greek alphabet, Bayer designed additional stars using Latin letters, and subsequent astronomers have enhanced the system to use pairs of letters (I find seemingly-conflicting descriptions of the ordering rules, so I haven't described them). These days, only brighter stars with Greek letters are much referred to by these designations. References often spell out the Greek letter, e.g., "Alpha" for "α". Also, the constellations are sometimes abbreviated. Thus the following equivalent designations:
I've heard of references using the upper case Greek letter, i.e., "Α Ori" for α Ori, "Α" being upper case "α". This would be ambiguous if Latin-letter designations were still employed for additional stars. Presumably the Bayer designations still commonly used don't include these ambiguities.
In some cases, a superscripted number (2, 3, ...) on the Greek letter is used to designate another star, generally very near within the celestial sphere, e.g., Mu Octantis and Mu² Octantis.
Extra-solar planets are conventionally designated by a designator for the star followed by b, c, d, etc., for the first, second, and third planet discovered. For an example in which the star is known by its Bayer designation, "Upsilon Andromedae b" designates the first planet discovered that orbits the star "Upsilon Andromedae".