Hi Jack,
What would happen is, whenever a user creates a new twitter account, it would store the data in the twitteraccounts table, with the userid being the id linked to the specific user the accounts belong to.
So what you would do to extract the information from both tables which belong to one user, is use a join.
So you could have a query like this:
SELECT user.*, twitteraccounts.* FROM user INNER JOIN twitteraccounts ON twitteraccounts.userid = user.id WHERE user.id = 1
This would get all of the information from both tables, where the userid is equal to 1.
Hope this helps to illustrate the table relationships.
Brad