Hair dye is a chemical preparation designed to give colour to the hair, either to cover grey hair or to change the style. This procedure can be done at home, or in a salon (which is advisable if you are inexperienced, or if you need to bleach your hair beforehand).
There is a wide variety of shades, as well as brands and types of dyes, as some stay in the hair until it grows back, and others wash out after the first few washes. There are also natural, ammonia-free products, but they do not last long on the hair.
The biggest advantage of hair dyes is that they allow you to hide grey hair, giving you a fresher, more youthful look. They also give you the ability to create new looks, and change your style as often as you like, as there are plenty of shades to choose from.