The picaresque is a genre of novel that evolved in Renaissance Spain. It is named after the protagonist, a "picaro", or likeable rogue. Rather than having a closed plot, it is structured episodically, with the protagonist falling in and out of scrapes of various sorts. Although the picaro often ends up on the wrong side of the law or of social convention, he or she generally maintains a degree of personal integrity, and is a sympathetic character. In other words, the picaro may con a bit of money from a rich merchant or have an occasional illicit sexual relationship, but doesn't torture people or kill babies. The first major picaresque was Lazarillo de Tormes. The genre continues ion the Continent and in England, with Tom Jones and Moll Flanders (with a female picaro) being well known examples.
a reasonably competent answer
in even simpler terms, we can say that the 'picaresque' novel was an early type of novel which related the adventures and deeds of a rogue or such type of person.
as stated above ''Lazarillo de Tormes'' (1554), is supposed to be the earliest picaresque novel, from Spain-- some good examples of such novels in English are also given above, although it is generally supposed that Thomas Nashe's ''The Unfortunate Traveller; or The Life of Jack Wilton'' (1594) was the first in this language.