A database table with a minimal amount of data for each site that lives in that database.

the SITE_KEY, used to identify content and users that are kept in the same database.

specifies what objekt is the front page of the site.
This is done with a drop down menu, and could get too big. In yoursite/siteclassess/confs/sites.conf you may specify an array of classes to limit your possibilities to.

the default Layouts for the site, used for most page displays unless the objekt class implements pageLayout() to return a different one. phpsnippet does this.

You should not create a new site record yourself, but allow the install.php script to do that for you. You may manually delete them, and you can certainly edit them.

see Installing a New Site