Cookies are just small, often encrypted, text files created by websites that are stored on your computer. These text files dont contain any code, just data, so they can't contain a virus or spyware.
A website will usually store information in this cookie such as your last visited page, when you last logged in and if you are logged in just now. This all helps to give you a better browsing experience, these are known as Session Cookies.
Usually no, they cant.
But sometimes where the websites all belong to the same domain, for example
These are all sub domains of cookies.com, and if the cookie has been set using '.cookies.com' as its 'root domain' then they can all share the same cookie, First Party Cookies.
There are 4 types of cookies that websites use:
First Party Cookie: These are cookies that are restricted to the one domain and are only readable by that domain.
Third Party Cookie: Typically used by analytics and tracking services, where the cookie isnt set by the website you are browsing but by another domain such as example-analytics.net, it will be used to track visits to that website but can also track your journey through the website. If you visit any other websites using the same analytics service it can track you between them.
Persistent Cookie: These expire after a fixed date, for example after 1 month, and are not cleared when the browser closes. Most commonly used for those "Keep me logged in" boxes.
Session Cookie: These expire when the browser is closed or after a shorter period of time, for example 10 minutes, they are typically used to remember if you are currently logged in to the website.
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