The Taliban are members of a movement primarily located in Afghanistan, but also spilling over into neighboring Pakistan. They are comprised mainly of members of the Pashtun tribe, and adhere to a strict interpretation of Sunni Islam, which is strongly anti-western, opposed to the use of alcohol, restricts the role of women, and imposes many restrictions on the personal lives of its followers with respect to clothing, entertainment, and social interactions.
The history of the Taliban is complex. The term and the movement per se first emerged in the 1990s. The origins of the Taliban, though, lie in an earlier group, the "mujahideen," who were Islamist freedom fighters opposing Soviet-backed government, with support from the United States and other western countries. The Taliban assumed power after the withdrawal of Soviet forces, and established an Islamic caliphate. Taliban rule was overthrown by western forces in 2001, but the Taliban has continued to be active in Pakistan and Afghanistan.
This complex history means that it is difficult to pinpoint the nature of the Taliban. They are often described as "radical Islamists" in light of their ideology. Given that they are not legitimate members of the current government, they can be described as "militants" belong to an "insurgency." In the west, they are often called "terrorists" due to the casualties they inflict upon civilians. They often consider themselves, and are considered by some fellow Islamists, as "freedom fighters" or even a legitimate government illegally overthrown by foreign powers.
Perhaps the most neutral term would simply be "Islamic movement," as it does not convey either negative or positive overtones.