The four great streams of Hinduism are filed here as denominations, and they are not denominations. The category is doing work it was not built for, and saying so is more useful than pretending otherwise.
A denomination, in the sense the word carries out of Western Christianity, is a body that separated over a dispute, governs itself through a polity, and holds an exclusive membership: one belongs to it and therefore not to the others. A sampradaya is a tradition of transmission, a line of teachers, a body of texts, a preferred deity, and a way of approaching that deity. It has no register of members, no boundary that must be crossed, and no mechanism of expulsion. A Hindu can and routinely does keep a Vaishnava family deity, visit a Shiva temple, observe Navaratri for the Goddess, and regard none of that as inconsistent. These are orientations rather than allegiances, and the population figures given for them are modelled from other data rather than counted.
Vaishnavism is the largest of the four: the worship of Vishnu as the supreme reality, and above all of his descents into the world, the avatars, of whom Krishna and Rama are the great ones. Its characteristic mode is bhakti, loving devotion to a personal Lord who is gracious, who can be addressed, and who responds. That is a real theological difference from the impersonal absolute of Advaita, and Vaishnava philosophers have argued it against the Advaitins for a thousand years without either side conceding an inch.