In this blog I will discuss, what an external user is and how a Sharepoint site can be shared to external users.
External User
External user means, a user who has an Office365 subscription but is not under your domain. Suppose for example, abc.onmicrosoft.com is one domain wanting to give access of one of its sites to users of another domain, xyz.onmicrosoft.com, here all users under domain xyz will be considered as external users.
Note
For demo purposes I have used my two different trial accounts. To enable external site sharing follow these steps,