What is the nature of the relationship between a guru and their disciple?

From ancient times until today, the art and science of yoga has been handed down from guru (teacher) to disciple (in Sanskrit shishya or chela). Disciples in turn hand down what they have learned to their disciples. Every guru is also simultaneously a disciple not only of their immediate guru, but also of all the preceding gurus in their line of teachers. In this way, the yoga/Hindu Vaishnava culture has been handed down from time immemorial.