Understand the Super key, primary key, Candidate key and Foreign key

Primary key:
Primary key is one of the attribute of a relation that uniquely identifies a row on a touple.
Super key:
Super key is a combination of attributes of a relation that uniquely identify a database table.
Candidate key or alternate key:
Candidate key is a subset of super key. A candidate key is a single field on the least or minimum combination of fields that uniquely identifies each record in the table.
Foreign key:
A relation schema may have an attribute that correspond to the primary key of another relation. The attribute is called foreign key.
Example:
To clearly understand the database key I discuss it with example. See the example below

	Student
student_id	first_name	last_name	course_id
A1	Kabir	Rock	L1
A2	Rahim	Doc	L2
A3	John	Bon	L3
A4	Bela	Din	L4

Consider the relation student. In that relation we uniquely identify the student using student_id, so it is primary key of this relation.
The combination of student_id, first_name and last_name is a super key but only first_name is not super key because first_name not uniquely identify the relation.
Candidate key is a minimum set of super key that uniquely identify the relation. So student_id would be a candidate key and (first_name, last_name)  set would be a candidate key. 

course
course_id	course_name
L1	S1
L2	S2
L3	S3
L4	S4

Now consider another relation course. In that relation course_id is the primary key but in relation student it is  used as a foreign key.