Tantra

Tantra is confusing to observers, because it looks like polytheism. However, it is not. It's enormously important for tantra to be approached within an understanding of shunyata, or it devolves into nothing but polytheistic god-worship. Within an understanding of shunyata, the deities are neither real nor not-real; they find existence in what they evoke within the practitioner.
In popular culture, tantra is associated with sexual practices. This may be because much of the imagery associated with tantra is highly sexual; copulating deities, for example. The actual sexual practices of religious tantra, however, are supposed to be free of lust and mostly involve visualizations.
Although tantra is associated with the Vajrayana practices of Tibetan Buddhism, there are elements of tantric practice in most schools of Mahayana Buddhism.