Some useful definitions...
...for things we might not have been explicit about elsewhere.
API - "Application Programming Interface" - these are scripts written so developers can tell the calendar what to do without getting deep into all the scripts that do it. =) The main API in our calendar is the script "Create Edit Delete Event ( SourceN..." More on the idea of an API from wikipedia.
Foreign Key - the primary key from another table, used inside a child table to link records together. So if we have Invoices and Invoice Line Items, the line items table contains a field for the invoice id: that is the foreign key in the line items table.
Primary Key - this is the unique ID or serial number for your record. A contacts's ID in a contact's database. This is probably field you're already using to related tables to each other.
TO - short for "table occurrence" - a representation of a table on the "Relationships" tab of "Manage / Database"