Difference Between Onchange and OnSelectedIndexChanged

Question

H Friends......,

What is the Use of OnChange and Use of OnSelectedIndexChanged

And What is the Difference between them

I need both onchange in javascript and OnSelectedIndexChanged in Asp.Net to Work on Particular Dropdownlist

if i use onchange & OnSelectedIndexChanged is not working on Particular Dropdownlist.  i've Used Autopostback=true also, what is the problem i've googled it i can't find clear solution

Can in get Clear Difference and Solution
ASAP

With Regards.....,
R.V.Rajagopalan