"Sexting" is when people send sexual messages – sometimes together with photos or videos (also known as nude or semi-nude selfies) – by text, an app or online.
People might send sext messages to boyfriends, girlfriends, someone they fancy, someone they've met online, or a friend for a laugh.
According to sexual health and wellbeing charity Brook, some young people feel under pressure to swap personal pictures and messages because they think everyone else is doing it.
But research shows they're not. And sometimes sexting can be harmful.
Sexting images is illegal for under-18s
Sending – or even possessing – sexual images or videos of anyone under the age of 18 is illegal, even if the participants agreed to it.
As Brook explains, this is classed as a form of child abuse or child sexual exploitation.
But research shows only a small minority of people under the age of 16 are sexting.
Why do people try sexting?
Some people might like the idea of sexting rather than talking face to face.
Lots of people find it easier to say what they really feel, what they really like and what they really want by text, email or instant messaging.
For many people, talking online is a part of everyday life, as is sharing photos through social media.
So sending a sext might feel like only a small step, especially if it's to someone you're flirting with or who you fancy.
What can go wrong with sexting?
Once you hit "send", the message or picture is out of your hands. It could be seen by anyone, including your friends, family or total strangers.
If you send or upload a picture and then regret doing it, there's no guarantee you can get it removed. Even if it can be deleted, it could already have been copied.
Similarly, if you send a picture or video to someone but then ask them to delete it, they might not want to, may not know how to, or might already have shared it with other people or saved it elsewhere.
Sexts shared with other people or uploaded on to websites without your permission is a form of cyberbullying.
It can lead to threats being made – for example, your family will be shown the pictures if you do not send more images.
At its most extreme this is sometimes known as "sextortion".
Is sexting harmful?
There are risks involved that can be harmful, as James, 17, discovered.
"Someone saw a video message I had sent to a previous girlfriend, took a screenshot and posted it online," he says.
"Lots of people I knew saw it and I was called a pervert. I was completely devastated and, to be honest, almost suicidal.
"I got the picture taken down eventually, but by that stage people had 'unfriended' me and the damage was done."
Grooming
Sometimes sexting can lead to "grooming" online by strangers pretending to be younger than they are.
This happened to Kathryn at the age of 14. Her sister Abigail, 17, met a stranger online who said he was also 17 – he was in fact 42.
Abigail and the stranger became involved in secret phone calls and sexts. The stranger also began to phone and sext Kathryn.
Fortunately, Kathryn's mother discovered one of the sexts and found out the man had arranged to meet both girls after school. The police intervened and the man admitted grooming Abigail, but not Kathryn.
Kathryn was devastated. She had been through the same experience as her sister, but felt she wasn't believed by anyone apart from her family.
She felt it destroyed her self-esteem and started self-harming. Through the support of the NSPCC and her family, Kathryn has recovered, but she feels sexting is "madness".
"I know sexting seems 'normal' to most people of my age. Because of what happened to me, I'm much more vigilant now about how people can use and abuse you."
How can you avoid people seeing your pictures?
There is very little you can do to stop people seeing your pictures or videos once they're uploaded online or sent to someone else.
Some apps, like Snapchat, will automatically destroy pictures after a set period (up to 10 seconds) and will tell you if someone takes a screenshot. But people can still take pictures of pictures with other devices or apps.
If the picture has been shared on social media or online, you can usually ask the service provider to remove it, but it can take time, and it’s not always possible.
You can report it to the Child Exploitation and Online Protection Centre (CEOP). CEOP will help you access support and advice.
For more advice about how to stay safe, visit the CEOP Thinkuknow website.
If someone you know suggests sexting
Remember that relationships should be based on respect and trust. If someone truly respects you, they will not pressure you into doing something you don't want to do.
"If someone is making you feel uncomfortable," says Kathryn, "saying or asking things that just don't feel right, stop all contact immediately.
"Talk to your mum or dad or, if you don't feel it's something you can share with them, make sure you tell another trusted adult or a service like Childline.
"Don't try to deal with it alone."
James, Abigail and Kathryn's names have been changed
Your health, your choices