Primary keys are the unique identifiers for each row. They 
must contain unique values and cannot be null. Due to their 
importance in relational databases, Primary keys are the 
most fundamental of all keys and constraints. A table can 
have only one Primary key.
Foreign keys are both a method of ensuring data integrity 
and a manifestation of the relationship between tables.

Primary key is a unique id for each table. This prevents 
duplication of records. Each table can have a single 
primary key. They can be refrenced by any tables as a 
foreign key.

Primary keys are by default clustered index.