A People Group is an ethnolinguistic group with a common self-identity that is shared by the various members. There are two parts to that word: ethno and linguistic . Language is a primary and dominant identifying factor of a people group. But there are other factors that determine or are associated with ethnicity.
Usually there is a common self-name and a sense of common identity of individuals identified with the group. A common history, customs, family and clan identities , as well as marriage rules and practices, age-grades and other obligation covenants, and inheritance patterns and rules are some of the common ethnic factors defining or distinguishing a people. What they call themselves may vary at different levels of identity, or among various sub-groups.
For strategic purposes, a people group is the largest group through which the gospel can flow without encountering significant barriers of understanding and acceptance.
An Unreached People Group is an identifiable group of people distinguished by a distinct culture, language, or social class who lack a community of Christians able to evangelize them without outside help. Their only opportunity to hear about salvation is through an "external witness." It is estimated that of the 6 billion people in the world today, 1.2 to 1.4 billion have never heard the gospel of Jesus Christ. There are about 6500 unreached people groups in the world today and 639 of those are unengaged.
For a comprehensive list of all of the unreached people groups of the world, check out the Joshua Project website.