As stated in the about page, Tails is designed to leave no trace on the computer you're using unless you ask it explicitly. It is important to understand some of the consequences of that.

Starting a computer on a media containing Tails doesn't change anything on the operating system actually installed on your hard drive: as a live system, Tails doesn't need to use your hard drive during the whole session. Be your hard drive absent or damaged, it wouldn't prevent your computer to start Tails. Consequently, removing the DVD or USB stick containing Tails is enough to retrieve your usual operating system.

You should save anything you want to keep for later access into a separate device (other USB stick, other DVD or any device you would choose), or use the persistence feature.