According to wikipedia, 'Writing is the representation of language in a textual medium through the use of a set of signs or symbols (known as a writing system).'
Along with reading, writing is one of the many educational type hobbies and activities a person can do. You can write about your life, places you've visited, things you've seen, your emotions, stories or homework and work documents.
Many people don't see writing as something enjoyable, however it can be extremly enjoyable and time consuming when it comes to writing about something you admire or great ideas you have that come across your head.
Try, for example on a rainy day, to write about what you wish you were doing at that moment and then from that you'll see different ideas come through your head and you could end up writing a small story. You can even earn money by sending these stories off to freelance companies, or if you reach the point of writing a mini novel, you can get your work published and you might become the next Roald Dahl.