Salesforce URLFOR()

URLFOR() function in Salesforce is not widely used as it is only available in custom buttons, links, s-controls, and Visualforce pages.

Syntax: {!URLFOR(target, id, [inputs], [no override])}
target: URL or action, s-control, or static resource merge variable
id: a reference to the record (depends on the “target”)
inputs: optional parameters with format: [param1="A", param2="B"]
no override: optional boolean flag - default false. Set to true to display a standard Salesforce page, regardless of whether you have defined an override for it elsewhere.

 The most common URLFOR() is used by developers in Visualforce pages to refer to a resource in Static resources, such as images or scripts.

Visualforce
<apex:image url="{!$Resource.ImagePba}" width="50" height="50" />
This ImagePba in the sample above is the Static Resource name, not the original image file name.

Optionally, you can add URLFOR()
<apex:image url="{!URLFOR($Resource.ImagePba)}" width="50" height="50" />

But when you use the resource in an archive file (such as a .zip or .jar file) in a static resource, URLFOR() is a must as a second parameter.
<apex:image url="{!URLFOR($Resource.Images,'pba.png')}" width="75" height="75" />
<apex:image url="{!URLFOR($Resource.ImagesFolder,'image/campaign1.png')}" width="100" height="100" />
<apex:includeScript value="{!URLFOR($Resource.LibraryJS, '/base/subdir/file.js')}"/>


Custom Button or Link
Admin can make use of URLFOR() function with $Action global variable in custom Button or Link. The $Action global variable provides methods such as View, Clone, Edit, and Delete. Some objects support other additional actions, see all valid actions here.

URLFOR() determines your base URL, and the $Action determines what view of a record to go to (the view page, edit page, etc.). For example, to edit the Account from the Contact page, add the custom button or link in Contact: {!URLFOR($Action.Account.View, Account.Id)}.

More samples
New Account -- {!URLFOR($Action.Account.New)}
View Account -- {!URLFOR($Action.Account.View, Account.Id)}
Edit Account -- {!URLFOR($Action.Account.Edit, Account.Id)}
Clone Contact -- {!URLFOR($Action.Contact.Clone, Contact.Id)}
Account Tab -- {!URLFOR($Action.Account.Tab, $ObjectType.Account)}

Reference:

• Formula Operators and Functions I–Z
• Referencing a Static Resource in Visualforce Markup
• Valid Values for the $Action Global Variable
• Constructing Effective Custom URL Buttons and Links