difference between and constraints and triggers

Constraints are used to maintain the integrity and atomicity of database.
In other words it can be said they are used to prevent invalid data entry.
The main 5 constraints:

1. NOT NULL

2. PRIMARY KEY

3. FOREIGN KEY

4. UNIQUE KEY

5. CHECK

Triggers are basically stored procedures, which automatically fired when any insert, update
or delete is issued on table.

Trigger effected only those row after which trigger applied but constraint effected all row of table.