They were especially ruthless bandits, who invariably murdered their victims before robbing them, but not – as they are usually presented – hereditary bands of religious fanatics offering human sacrifices to the goddess Kali. I touched on the key points in this earlier response:

What were Kali-worshipping Thuggees really like compared to how they're portrayed in Indiana Jones: Temple of Doom?

Note that, while in my opinion there is decent evidence that Thug groups did actually exist, a significant group of postcolonial lit scholars have argued fiercely that they were nothing but the invention of the British colonial administration.

It's a tangent but in 19th century Britain (mostly London it seems) there was a garroting panic where people believed criminals were going around strangling people in a similar way to how it was reported Indian thugs/thuggee had done. The comparison was actually made at the time, especially in newspapers which loved to sensationalise these events.