There are 3 String Quartets in Beethoven's Op 59. These were commissioned by Count Razumovsky. These quartets are usually included with Op. 74 "the Harp" and Op. 95 as the "Middle Quartets"

The String Quartets was created in 1998-12.

Haydn wrote 84 string quartets.

Maybe you mean 'What are Haydn quartets' They are string quartets.

Haydn's quartets Opus,17, 20, and especially Op.33. In1782, Mozart also began a thorough study of the works of Bach and Handel at the urging of Baron van Swieten. These did not include any string quartets, yet their influence on the "Haydn" quartets is obvious and strong. Of course, Mozart was also acquainted with the quartets of Richter, Dittersdorf, Vanhal, and the other Mannheim and Viennese composers, but in the "Haydn" quartets he is moving away from their ideas.