Follow this lick to get answer....

http://msdn.microsoft.com/en-us/library/aa933117.aspx
http://www.1keydata.com/sql/sql-foreign-key.html

thx

A foreign key is a field (or fields) that points to the 
primary key of another table. The purpose of the foreign 
key is to ensure referential integrity of the data. In 
other words, only values that are supposed to appear in the 
database are permitted. 

For example, say we have two tables, a CUSTOMER table that 
includes all customer data, and an ORDERS table that 
includes all customer orders. The constraint here is that 
all orders must be associated with a customer that is 
already in the CUSTOMER table. In this case, we will place 
a foreign key on the ORDERS table and have it relate to the 
primary key of the CUSTOMER table. This way, we can ensure 
that all orders in the ORDERS table are related to a 
customer in the CUSTOMER table. In other words, the ORDERS 
table cannot contain information on a customer that is not 
in the CUSTOMER table. 

The structure of these two tables will be as follows: 

Table CUSTOMER column name characteristic 
SID Primary Key 
Last_Name    
First_Name    


Table ORDERS column name characteristic 
Order_ID Primary Key 
Order_Date    
Customer_SID Foreign Key 
Amount    


In the above example, the Customer_SID column in the ORDERS 
table is a foreign key pointing to the SID column in the 
CUSTOMER table.

It is Reference to another table