Erik Eriksen describes life as a series of conflicts or tasks that we have to face in order. If we don't resolve the conflict/accomplish the task of each stage, then we won't be able to successfully move on to the next one.
His theory has been criticized for being ethnocentric, but it has always made a lot of sense to me.
According to him, the conflict of adolesence is between identity and role confusion. In other words, teenagers have the experience of feeling like different people at different times and spend a lot of time experimenting with different identities. In order for identity to win out, you have to develop a sense of who you are that remains stable even when you find yourself in different situations and with different feelings.
After that, you're ready for the next conflict--intimacy vs. isolation. In early adulthood, now that you know who you are, you have to figure out how you're going to relate yourself to other people and how to develop intimate relationships with them.