User Exits

Hi Gurus,

I search the forums for the difference between the userexit and customer exit. But I dont have clear idea for that. What I know is

1) For userexit the exit is stored in the SAP namespace and for customer exit exit is stored in the customer namespace. Please clarify me if it is right or wrong?

2) for userexit we need a access key and for customer exit I think we dont need access key for implementing the exit. Please clarify me in this?

3) In customer exits like screen exit, menu exit, Function exit and Field exit which exit we need access key to implement?

<b>Points will be awarded</b>

Thanks in advance

Ravi