Prakrits are various Indic languages and dialects that developed in medieval times from Sanskrit. The most popular among them was Pali. Typically they were vernacular languages, often used for ordinary speech while Sanskrit continued to be used as the literary language; some of the Prakrits, however, developed literary languages of their own. We might say that the Prakrits are to Sanskrit as Vulgar Latin and the Romance languages are to Classical Latin.
Some scholars include all modern Indic languages ultimately derived from Sanskrit under the rubric of "Prakrits"; others prefer to designate as Prakrits only dialects and languages that has been used in medieval times, and to consider modern Indic languages as having developed from Prakrits, starting from around 1,000 - 1,200 CE.