The core report is short. Asked about it, the Prophet is recorded as saying that every person has a companion appointed from among the jinn, and when the companions asked whether this applied to him as well, he answered that it did, but that God had helped him against his, so that it submitted. The Arabic of that last clause carries two readings, one in which the companion itself became Muslim and one in which the Prophet was made safe from it, and both are discussed in the commentary tradition rather than one being obviously correct.
Around that report a large body of folk practice grew, and it varies widely by region. The qarin is held responsible for whispered suggestions, for bad dreams, sometimes for a person's more troubling impulses, and in some traditions a person has a qarin of the opposite sex. Practitioners in several parts of the Muslim world treat the qarin in the context of healing and protection, and those traditions are living and are practised now.
The distinction worth holding is between the scriptural and hadith material, which is narrow and specific, and the folk elaboration, which is broad and local. Western writing on the subject regularly presents the second as though it were the first. An atlas can record that both exist, that they are not the same, and that the practical traditions belong to the communities that keep them.