Mujahideen are a group of people who fight on behalf of Islam. Most modern-day mujahideen are fundamentalist guerrilla fighters.
A mujahid is any Muslim who performs jihad. Today, both terms carry connotations of violence and extremism, but the original meanings were quite different. In Arabic, mujahideen are "strivers" seeking right conduct or Godly rule. Jihads originally included "right conduct" like giving money to poor people. In the 20th century, mujahideen became associated with guerrilla fighters who defended Afghanistan against Soviet invasion in the 1980s, and today it describes fundamentalist warriors who engage in terrorist activities and non-Muslim violence.