"Slytherin in possession -- Flint with the Quaffle -- passes Spinnet -- passes Bell -- hit hard in the face by a Bludger, hope it broke his nose -- only joking, Professor...."
-- Lee Jordan, commentating the Gryffindor v Slytherin match (PS11)
The Quidditch commentator has the important role during matches of keeping the crowd up to date on what is happening all over the Quidditch pitch.
Lee Jordan of Gryffindor commentates the Quidditch matches at Hogwarts, using a magical megaphone to make his voice heard over the entire stadium. He attempts to be impartial, but does add his personal opinions to his commentary on everything from the model of broomstick being flown to the fact that Angelina Johnson won't go out with him. Lee gets rather carried away, particularly when Gryffindor is playing Slytherin. Professor McGonagall is obliged to stand by and threaten to take the megaphone away if his comments get out of hand (PS11, CS10, PA13, PA15, OP19, OP30).
After Lee has left Hogwarts, a Hufflepuff student, Zacharias Smith, took over commentating with the magical megaphone for the Gryffindor v Slytherin match. He was also biased - mainly against Gryffindor (HBP14).
Later that year, while Zacharias was playing as a Chaser in the Hufflepuff v Gryffindor match, Luna Lovegood took over as the Quidditch commentator. Luna's commentary was noted for being only partly relevant to events during the match and was, at times, completely off the wall. She hilariously blames one failure on the imaginary illness "Loser's Lurgy" (HBP19, HBP20).
Ludo Bagman commentates the 1994 Quidditch World Cup in a very professional manner. Rather than a megaphone, he uses magic on his throat "Sonorus" to make his voice heard and the incantation "Quietus" when finished (GF7, GF8). He later used the same magic spells to commentate the Triwizard Tournament at Hogwarts.