Almost all languages spoken today are related to at least one other, whether closely (like Spanish & Portuguese) or distantly (like English & Hindi). There are several different language families. However, there are a few languages that have no known relatives, called 'language isolates'. These are either languages whose 'missing links' to other languages have been totally lost, or that evolved in an isolated group of people with no other language.
The most well-known language isolates include Korean, Basque (areas of France & Spain), Zuni (New Mexico, US), & Ainu (Hokkaido Island, Japan).