Buddhist Tantra begins where Sutrayana ends: at emptiness. Tantra concerns the realms beyond emptiness, about which mainstream Buddhism (Sutrayana) has nothing to say.
Few Buddhist systems go beyond emptiness. This post is a humorous sketch of the differences among three: Zen, Tantra, and Dzogchen. (Mahamudra is another, which I won’t discuss.) I can’t write seriously, because my practice of Tantra and Dzogchen is pathetic, and I haven’t practiced Zen at all. The post is long, but I hope you will find it entertaining, and that it conveys something of the attitudes of the three approaches.