So tantra is not only Hindu and it is not only Buddhist. It does not include all of Buddhism, nor all of Hinduism. It is a movement that spread within those religions among some followers.
Tantric practices put an emphasis on mystical union between the devotee and a chosen deity.
We say “mystical” because we mean that this divine union occurs through various magical incantations, physical postures, and ritual implements (such as images of deities and mandalas).