Session cookies only retain information about your activities during your visit to the site. Once you close the browser, the session cookies are lost and the site will not recognise you the next time you return to the site. Session cookies help websites to recognise you and remember the information provided by you as you move from one page to another within the same website. If you have to login to a website every time you open your browser and visit it - then it is using a session cookie to store your login credentials. Many websites use session cookies for essential site functions, and to make sure pages are sent to the browser as quickly and efficiently as possible.
Persistent cookies enable websites to remember your preferences and settings (i.e. login information, language selection, font size preference, etc) so that they can offer you a more personalized and convenient access the next time you visit. For security purposes, your login information is generally encrypted by the web server before it gets stored in a cookie. Persistent cookies can exist for an extended period of time until expired or until they are deleted. They enable the site to recognise you on a continuous basis. This is done by the web server storing a small text file with a unique ID tag on your computer. On subsequent visits to the site, your browser delivers this cookie over to the site, allowing the site to retrieve the matching file. If you login into a website, then shut down your computer, start it up again, and go back to the website to find you are still logged in - then it is using a persistent cookie to remember you.