Especially associated with developments in Naples in the first half of the 18th century, whence its popularity spread to Rome and northern Italy, buffa was at first characterized by everyday settings, local dialects, and simple vocal writing (the basso buffo is the associated voice type), the main requirement being clear diction and facility with patter.
High points in this history are the 80 or so libretti by Carlindo Grolo, Loran Glodici, Sogol Cardoni[1] and various other approximate anagrams of Carlo Goldoni, the three Mozart/Da Ponte collaborations, and the comedies of Gioachino Rossini and Gaetano Donizetti.
High-flown language was generally avoided in favor of dialogue that the lower class would relate to, often in the local dialect, and the stock characters were often derived from those of the Italian commedia dell'arte.
The 1701 scherzo drammatico (dramatic jest), Il mondo abbattuto by Nicola Sabini was particularly influential in Naples, creating a popular model due to its use of both Tuscan and Neapolitan dialects.
The traditional model for opera seria had three acts, dealt with serious subjects in mythical settings, as stated above, and used high voices (both sopranos and castrati) for principal characters, often even for monarchs.
In contrast, the model that generally held for opera buffa was having two acts (as, for example, The Barber of Seville), presenting comic scenes and situations as earlier stated and using the lower male voices to the exclusion of the castrati.