Join tables

I seriously dont get Table joining… at all…

I’m trying to do this simple practice.

Table users: id - username - password - active
Table data : id - userid - details - date

I read that there must be a common table that is link together…
so i thought i’d link table user id with table data userid…

purpose of this is to get the details of a username and echo it…

Any one can explain how this is done? with extreme simplicity I’m seriously lost in this!

Thanks a Million !

     , users.username
     , data.details 
  FROM users
  JOIN data
    ON data.userid =
    BY users.username
     , DESC