Hi, 

As Vinita said you have to research to understand as they are whole concepts and not just questions. 

However, i will try to short answer them may be it will help. 

1. what is fieldset ?

A:  A field set is a grouping of fields. For example, you could have a field set that contains fields describing a user's first name, middle name, last name, and business title. Field sets can be referenced on Visualforce pages dynamically. If the page is added to a managed package, administrators can add, remove, or reorder fields in a field set to modify the fields presented on the Visualforce page without modifying any code.

http://www.salesforce.com/us/developer/docs/api_meta/index_Left.htm#CSHID=meta_fieldset.htm|StartTopic=Content%2Fmeta_fieldset.htm|SkinName=webhelp

2. Explain me what is customer portal & partner portal after creating the portals how to give access?

A: Customer portal is like self service portal where Customers can see thier cases. Partner portal is where we can share our cases(Objects) with our outsourced partner.  

3. example of junction object? where we r using this object? with example

A: Object used for creating many-to-many(m-m) relationship between 2 objects. Junction object will have 2 master-detail fields.

4. formula field data stores in database or not?

A: It does not store any values, the formula is run and the value is fetched only when the page is loaded. 

5. what is oppurtunity line items ? why we are using..

A: To keep it simple - its like the list of all the items that included in a package shipment. (laptop is a product - charger mouse additional battery are line items)

http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_objects_opportunitylineitem.htm

6. trigger.new/ trigger.old what is the differnce

A: 

Trigger.new : Returns a list of the new versions of the sObject records. Note that this sObject list is only available in insert and update triggers, and the records can only be modified in before triggers.

Trigger.old : Returns a list of the old versions of the sObject records. Note that this sObject list is only available in update and delete triggers.

7.Difference between pagelayouts & recordtypes.

A: Page layout: the design how we organise our fields based on sections, catagory, columns and sequence. 

Record type: for a certain record type only certain fields are displayed. (Ex: Car record type has steering fields in it and bike record type has handle fields in it.)

You would learn more my googling the topics and finding the Salesforce articles, however if there is something complex let us know we would be happy to clear any doubts. 

Regards,

Ashish