What Are Global Variables Explain With Examples?

Adding some points:

Global variable is a type of merge field that yields information about the organization or the current user of the platform. Each global variable is easily identified because it distinctly starts with a dollar sign “$”.

Global variables are used in conjunction with other merge fields to reference important data about the organization. Customized links, formulas, buttons, and Visualforce pages allow the use of various global variables.

Thanks

Refer this ex:

<apex:page controller="RecentlyViewedController">
<apex:pageBlock>

<apex:pageBlockTable data="{!RecentlyViewed}" var="rv">
<apex:column headerValue="{!$ObjectType.RecentlyViewed.fields.Name.Label}">
<apex:outputLink value="{!URLFOR($Action[rv.Type].View,rv.Id)}">
<apex:outputText value="{!rv.Name}" />
</apex:outputLink>
</apex:column>
</apex:pageBlockTable>

</apex:pageBlock>
</apex:page>