Describing them as "fake penises" can be a bit misleading, because while some are designed to be penis-shaped, others are designed to look like dolphins or rocketships or goddesses or waves or abstract shapes or almost anything that the human mind can come up with . And it kind of implies that they're only used as "penis substitutes", which isn't the case.
Basically, they're sex toys used for vaginal (or anal) penetration.
Copyright 1998, 2014 Heather Corinna/Scarleteen
Scarleteen.com: Providing comprehensive sex education online to teens and young adults worldwide since 1998
Information on this site is provided for educational purposes. It is not meant to and cannot substitute for advice or care provided by an in-person medical professional. The information contained herein is not meant to be used to diagnose or treat a health problem or disease, or for prescribing any medication. You should always consult your own healthcare provider if you have a health problem or medical condition.