Players take turns asking other participants "Truth or Dare?" This gives the participant the choice of answering a question honestly or performing a dare that the questioner provides. This is a popular game for people at slumber parties, at summer camp or when sitting around with a group of friends.
When asking a "Truth" question, ask something that elicits a deep response and gives you information you didn't know. Questions could include: "What do you find most and least attractive in a man or woman?" or "What is the most embarrassing thing that has happened to you?" The onus is on the person answering the question to be honest in answering, which is what makes the game fun.
If the person chooses "Dare," the limits of the dare really depend on the mores of the group that you're with. Some samples include a dare to place a prank call on someone that the group knows, to eat six tablespoons of ranch dressing, to drink a certain number of shots, to stick a body part into the water in the toilet bowl, and so on. You can decide as a group whether to allow people to change from Truth to Dare or vice versa after hearing the question or the dare.