A diffuser is an attachment that goes on the end of your hairdryer. It diffuses the air as it come out so that you no longer get a blast of air on your hair, but rather it spreads softly through your hair. Diffusers are mainly used to style curly hair as it lets your dry your curly quickly without disturbing their natural shape.
There are two main types of diffusers. The round plastic attachments, as shown in the photos, which are custom made for a particular hairdryer model or you can buy a plastic universal diffuser that fits most hairdryers.
The other type of diffuser available is called a sock diffuser. This is made of a fine heat resistant mesh and fits like a loose sock on the end of your hairdryer nozzle. The sock diffuser is great for travelling as it is very small and easy to pack. (It also won’t freak out the guys as customs like my plastic diffuser does!) It’s also the only option for hairdryers that do not fit a universal diffuser. :)