1. How to have standard object as required child to custom object(which is not possible through standard process, have to bypass this restriction)?
Create Lookup and make the lookup field mandatory.

2. In a visual force page the save should ensure the data to be be stored in current object as well as associated child object?
We have to use Database.SavePoint and Database.Rollback for this situation.

3. what is audit field and what is the purpose of audit field?
Created By, Created Date, Last Modified By and Last Modified Date are audit fields. Used to track when the changes are done to the records.

4. what we need to do for extending the limit of creating only 2 M-D relationships for custom object?
Create Lookup and make the lookup field mandatory.

5. How to write java script code for save button?
We have to create custom button and in that custom button we have to write Java script code.

6. What are the attributes of apex tag?
Attribute tag is used in creating components.

7. How to insert value to a parent and child element at the same time?
Use triggers.

8. What  is the use of record types?
http://infallibletechie.blogspot.com/2013/05/record-types-in-salesforce.html

9. How to make pick-list as required (thru java script)?
We have to create custom button and in that custom button we have to write Java script code to check whether the picklist value is null.

10. What is the difference between Validation rule and roll-up summary fields?
http://infallibletechie.blogspot.com/2012/09/roll-up-summary-field.html

11. What is the Difference between Ajax and ActionPoller?
ActionPolleris a timer that sends an AJAX update request to the server according to a time interval that you specify.