Let's say I make a social networking website like myspace. I was wondering when user's initially register to become members of myspace, myspace will send the registered user an e-mail saying something like "welcome, and what not"

I was wondering, does this registered persons' email get stored somewhere?
If it does, where does it get stored?

I know this is a theory question but I was just curious. Thanks.

Probably, it'll be stored inside a row somewhere in a DB table, it all depends on how you design the database.

Check the message table of your database .... which you are using