In the Star Trek universe, Tholians are an alien race. We see little of them, but what we do see reveals that they are short-tempered, fierce, violent, territorial, and unrelenting.
They appear to be red and green crystalline, non-humanoids with a hive-mind. Two Tholian ships can spin a web of energy (a tractor beam, essentially) in order to trap an enemy vessel, after which the web is then drawn together, constricting and finally destroying the vessel inside. On stardate 5693.2 (2268), they made first contact with the United Federation of Planets when the USS Enterprise (NCC-1701) supposedly violated their space when attempting to rescue the USS Defiant. (TOS: The Tholian Web)
There has since been an ongoing conflict between The Tholian Assembly and the United Federation of Planets which increased when the Tholians attacked a Federation starbase in 2353, killing almost all personnel. (TNG: The Icarus Factor)
Tensions lowed slightly when, in 2371, a Tholian ambassador visited Deep Space 9. (DS9: [Life Support)
Their homeoworld is Tholia II.