In fact a website cookie is a small data file that is automatically saved onto your device (whether that is a desktop computer, laptop or smart phone) when you are browsing a website. These files can allow a website to ‘remember’ who you are, in order to offer a more personalised web experience.
As an example, it is cookies that allow a website to ‘remember’ what is in your shopping basket as you navigate around the site. Without cookies your shopping cart would empty every time you browsed to a new webpage.
While a session cookie will expire after a certain time period (for instance when you close your browser) a persistent cookie will hang around persistently. The latter kind of cookie will allow the website to ‘remember’ the contents of your basket weeks or even months later.