Trigger

Question

helo sir , can u explain me any1 eaisly DMLTrigger wth Example @ wht's the diff b/w after & instead of trigger . How can be Update Insert & Delete Record...in this table..
Suppose we have to Update Record in Salary Against The Age.. (DELETE , Insert) plz Explain me  I have urgent Req

Employee

ID	NAME	Age	Salary	Department
1	A	20	15000	HR
2	B	21	2000	DOTNET
3	C	20	15000	JAVA
4	D	23	30000	HR