Primary key VS Unique key

Primary key - Primary key means main key
def:- A primary key is one which uniquely identifies a row 
of a table. this key does not allow null values and also 
does not allow duplicate values. for ex,
     empno   empname   salary
       1      firoz     35000
       2      basha     34000
       3      chintoo   40000

it will not the values as follows:
1      firoz      35000
1      basha      34000
       chintoo    35000

Unique key -  single and main key
            A unique is one which uniquely identifies a row 
of a table, but there is a difference like it will not 
allow duplicate values and it will any number of  allow 
null values(In oracle).
it allows only a single null value(In sql server 2000)

Both will function in a similar way but a slight difference 
will be there. So, decalaring it as a primary key is the 
best one.

foreign key - a foreign key is one which will refer to a 
primary key of another table

for ex,
       emp_table                         dept_table
empno empname salary  deptno         deptno deptname

In the above relation, deptno is there in emp_table which 
is a primary key of dept_table. that means, deptno is 
refering the dept_table.

Foreign Key: A foreign key is one or more columns whose
value must exist in the primary key of another table.