Commedia dell'Arte is improvisational comedy, and it was first in Italy around the 16th century. Many Historians believe this was a response to the economic and political happenings. Performers are given a scenario, but that is it. It’s basically in the form of an outline, and the performers have to go from there. A professional troupe usually had a week or so to create an entire show from directions like "Pantalone and Gratiano enter and something funny happens" or even as vague as "Everyone comes on stage and everything is resolved happily".
You may think that this is really easy to do, but it really isn’t. The troupe write the whole dialog, which also means they need to have the wit and imagination of a playwright.
Since the performance is improvised, there aren’t any cues that most people need and the timing is much more important than in regular theater.
Since it is comedy, the timing is unforgiving. A performer could be semi-tragic, and people will still feel. However, if the performer is only semi-funny the audience won’t laugh.
Characters are generally the same, but include the following. Pantalone is a merchant from Venice who is greedy and rich. Gratiano is usually a lawyer. Pantalone and Gratiano are referred to as the "vecchi", which is Italian for old male characters. Pedrolino and Arlecchino are the servants of Pantalone and Gratiano. Collectively they are known as zanni. In addition, most plays include Capitano (a “scaredy-cat” Spanish Captain), villains, and many others.
Because of all the things that go into these productions, and the difficulty, many historians consider this to be one of the first forms of professional theater.