Delta-8 tetrahydrocannabinol (THC) and delta-10 THC are the molecular siblings of the infamous delta-9 THC. The difference is the location of the double bond, where carbon atoms share two pairs of electrons. In delta-8, the double bond is located between carbon atoms 8 and 9; for delta-10, the double bond is found between carbon atoms 10 and 11 (see the difference in the images below). Both are elusive in cannabis/hemp, occurring only in tiny amounts naturally.
De Delta-8 THC is currently more popular and well-known. Hemp producers regularly convert cannabidiol (CBD) into delta-8 distillate through a process of isomerization. On the other hand, delta-10 THC remains rare. The laboratory of Raphael Mechoulam synthesized delta-10 THC (along with different isomers, including delta-2, delta-3, and delta-6) in the 1980s, but the process relied on harsh chemicals. 
Strangely, California cannabis producer Fusion Farms discovered delta-10 in batches that were accidentally contaminated with a fire retardant. Josh Jones, Ph.D., founder of Jonesing Labs LLC. and consultant for Fusion Farms, used a food-grade radical initiator to recreate delta-10. “Josh found that including catalytic amounts of a food-grade radical initiator, along with some other components to stabilize the reactivity of radicals, provided a 75-80% yield of Δ10-THC,” writes Jason Lupoi, Ph.D., in a recent interview for Extraction Magazine.
“The burning question yet to be discussed is what does Δ10-THC actually do, from a physiological and medicinal vantage point?” It is believed that delta-10 is similar to delta-9 THC but less potent; however, this is based on a 1988 study (also from Mechoulam’s lab) conducted on pigeons. 
On the other hand, more has been reported on the benefits of delta-8 THC. Like (hypothetically) delta-10 THC, it is less potent than delta-9 THC. Among many potential therapeutic benefits, delta-8 reduces nausea, stimulates appetite, exerts neuroprotective effects, and may have anti-cancer properties.
Despite the lack of understanding of delta-10, the interview on Extraction Magazine spread like wildfire (no fire retardant needed). Companies have latched onto delta-10 and begun producing distillate and vaporizer cartridges. How exactly the effects/benefits compare to delta-8 THC is still unknown. Both benefit from the legal gray area surrounding hemp derivatives, and some products now mix the two.
- Srebnik M, et al. Base-catalysed double-bond isomerizations of cannabinoids: structural and stereochemical aspects. Journal of the Chemical Society, Perkin Transactions 1. 1984: 2881-2886.
- Järbe T, et al. Separation of the discriminative stimulus effects of stereoisomers of A2- and A3-tetrahydrocannabinols in pigeons. European Journal of Pharmacology. 1988;156:361-366.