Tantra as I have experienced it is two things:
One: A very hard to find practice where your sexual experience is designed to be enhanced and deep.
Two: A easy to find trumped up excuse to charge more money by some sex workers.
The second definition is what people on here are posting about when they say it is 'just an expensive HJ'. Real Tantra can be really mind blowing, but it is next to impossible to find in a pay to play type scenario.
I had a 'Tantra' session just yesterday, and I can tell you it was a total joke. Total waste of time. My experience is that 'sex workers' are not very good at Tantra even when they want to be. They have too much experience charging money for sex, where the objective is to give the smallest amount of sex for the most money. Tantra is about having huge volumes of sexual stimulation which translates to huge cost.(in the mind of most sex workers)
For example:
Say you want to find a provider for a 10 minute HJ. That is easy. The worker might mentally think, "that is worth $200". Now try and find a provider with this type of mental math pounded into their head and ask, "how much for a 60 minute HJ?"
The worker will think 6 X 200, then realize that is too much and so will charge like $450, but only do a 30 minute HJ.
In other words the objective of a sex worker is to make the HJ as short as possible, and the objective of Tantra is the opposite.
But, to answer the question "what is Tantra?". It is a broad and deep philosophical approach to human sexuality. What is borrowed from Tantra and somewhat practiced by sex workers? Using teasing and sensual massage to slightly extend the "happy" part of the happy ending you might get at any rub and tug. So instead of, say 5-15 minutes of "happy", you might expect like maybe 20-30 minutes.