constraint is a rule which can not be voilated by end users.
Different types of constraints are available.They are:-
1)default constraint:-which is used to define a default 
value.
2)primary key:-the key which does not allows  duplication
and null values.
3)foreign key:-the key used to refer primary key defined 
field in another table and it allows duplication.
4)null:-which allows NULL values.
5)not null:-which does not allows NULL values.
6)unique key:-which does not allows duplication but allows 
NULL values.

Including these it also contains check constraint.
check constraint:-It specifies a range of values witn in 
which the attribute should take

Constraints are the constraints which can provide on the 
column data.The types of constraints are
1).Domain Integrity constraints
EX:Check
2).Entity Integrity constraints
Ex:Unique,Primary key
3).Referential Integrity constraints
EX:Foreignkey

Constraint : To Restrict the voilation of irrelegular data 
and maintain data integrity.

Primary key
Unique Key
NotNull
Forigen Key
Check

these are the constarints or codes which an organisation 
has to abide by. Not necessarily does any one have to abide 
by these, however there are many biles like that can kinda 
get peopel to abide by them u get me blud. Dun now this is 
dab real macoy doe.
Vote YES for dis 1 if u r a real G u get me.
All ady everyday 2K8 get 2 know.

. Primary Key Constraint
. Foreign Key Constraint
. Unique Key Constraint
. Check Constraint