For many years Birthright Israel has offered free trips to Israel for young adult Diaspora Jews aged 18-26. People are disqualified if they have lived in Israel for a certain amount of time or have already gone on a "peer trip" to Israel. But they changed the rules a few years ago to enable people who went on a trip while in high school to still qualify for a later Birthright trip. That meant that my kids who both went on teen group Israel trips when they were in high school could still qualify for a Birthright trip.
The main aspect about Birthright trips is that they are free to participants because the cost is paid for by wealthy donors. Here is the Wikipedia article about it: Birthright Wiki article
The trips are large group trips for young Jewish adults. Participants travel in chartered buses and do not stay with families. The Birthright program has a reputation of having trips filled with young adults who just want to party and sleep around because they are mainly attracted to the idea of a free trip to an exotic place. But there are different kinds of Birthright trips organized for different groups, so participants who have a serious interest in Israel should just be careful to choose an appropriate trip. Here is the Birthright website page showing the various types of trips: Birthright website And here is an interesting article by a participant about her experience: 'No Brainwashing!': A Skeptic Goes on Birthright