Wikipedia writes: "The Pakistani Inter-Services Intelligence and military are widely alleged by the international community to have provided support to the Taliban during their founding and time in power, and of continuing to support the Taliban during the insurgency." Also, Saudi Arabia provides financing.
Its strength was estimated to be around 60 thousand in 2014. That's up from 11 thousand in 2008.
The leader of the group appears now to be Sirajuddin Haqqani