Answer:
A foreign key is a type of constraint. In this example the value in a field must be the same as some value in a defined field in another table.
Example in a Customer Table you might have a Column (field) named StatusID
You would define a foreign key to the table Status, field StatusID.
The value in the Customer table, StatusID column must be an entry existing in the Status Table, StatusID column.
There are many constraints. The fact that a column can not be NULL (Left blank) is a constraint. Defining what KIND of data, or range of data that can be entered in a column is a constraint.