Field Sets in Slasforce

Field Sets is a new feature introduced in Spring 11 that allows Salesforce Admins to customize a Visualforce (aka VF) page via simple point-and-click. We can Create a Field Set on any Object that is the group of different fields.
Now this field Set is used in VF page with {!$ObjectType.ObjectName.FieldSets.NameOFFieldSet} in the VF page as follow-


Steps1- Create a Field Set on a object Suppose Account that include  Account Name, Account Owner, Billing and Shipping address fields(say AcctSearch).
Step 2- Now Use it in the VF page..
For more details See-
http://blog.sforce.com/sforce/2011/02/using-field-sets-in-spring-11.html

Param Tag In VF Page

<apex:param>

A parameter for the parent component. The < apex:param > component can only be a child of the following components:

• < apex:actionFunction >
• < apex:actionSupport >
• < apex:commandLink >
• < apex:outputLink >
• < apex:outputText >
• < flow:interview > so it work in between these tags only

With the help of param component, we can also define date format as below

<apex:page standardController="Account">
  <apex:outputText value="{0,date,dd MMMMMM yyyy}">
        <apex:param value="{!Account.SLAExpirationDate__c}" />
    </apex:outputText>
</apex:page>