What is a Portlet??

Portlet

• A Portlet is pluggable user interface jee web component which are configured,managed and displayed in Web Portal.
• A Portlet is generates a fragment of markup code that are aggregated into a Portal.
• A portal page is displayed as a collection of non-overlapping portlet windows, where each portlet window displays a portlet.
• Some examples of portlet applications are email, weather reports, discussion forums, and news.
• The Java Portlet Specification (JSR168, JSR286) enables interoperability for portlets between different web portals. 
• This specification defines a set of APIs for interaction between the portlet container and the portlet addressing the areas of personalization, presentation and security.
• Apache Pluto is the reference implementation of JSR168 and JSR 286. In addition to the reference implementation, a number of vendors provide commercial implementations of the portlet container. Some of the leading vendors are IBM, Oracle, OpenText Corporation (formerly Vignette Corporation), JBoss, and Software AG. 
• These vendors provide standards-based implementations as well as extensions not yet approved by the standards body. In addition, a number of open-source portal solutions support JSR168 and JSR 286, such as Apache's Jetspeed-2 Enterprise Portal, eXo Platform, GateIn Portal, uPortal and Liferay Portal.

What is Portal

Portal

• A Portal is generally defined as a software platform for building websites and web applications. 
• Modern portals have added multiple features that make them the best choice for a wide array of web applications.

Sothat we can use a Portal as a 

1.Web Platform
2.Integration Platform
3.Social Office
4.Collabration Platform
5.Web Content Management Platform

For more description refer this link:
http://www.liferay.com/products/what-is-a-portal/web-platform

What is Liferay Portal???

Liferay Portal

• Liferay is one of the free,open-source and enterprise portal written in java and constructed in the form of functional units called Portlets.
• Liferay  is JSR 286 enterprise portal which includes a suite of applications (e.g., Content Management System, blogs, instant messaging, message boards, etc.). 
• Liferay is sometimes described as a Content Management framework or a Web application framework. 
• Liferay's support for plugins extends into multiple programming languages, including support for PHP and Ruby portlets.

It is distributed in two different editions:

• Liferay Portal Community Edition — A version with the latest features and support through the active community.
• Liferay Portal Enterprise Edition — A commercial offering that includes services including updates and full support. This release goes through additional quality assurance cycles and is usually available around 1 or 2 months after the Community Edition and comes under a non-free license.

Liferay comes with certain portlets preinstalled.^[20] These comprise the core functionality of the portal system. They include:

• Alerts and Announcements
• Alfresco, Documentum, and other document library integration
• Asset Publishing
• Blogs and blog aggregation
• Breadcrumbs
• Calendar
• Chat
• Document and Image management
• Document Library Manager, Recent Documents
• Image Gallery
• Knowledge Base
• LDAP Integration
• Mail
• Message Boards
• Nested Portlets
• Page Ratings & Flags
• Polls
• Site Map
• Site Navigation
• Social Equity
• Software Catalog
• Tags and Categories
• Themes, supporting Velocity and FreeMarker markup
• User Directory
• Web Content
• Web Form Builder
• WebDAV Integration
• Website Tools
• Wiki (supports Creole as well as MediaWiki syntax)

for more reference go this link

http://en.wikipedia.org/wiki/Liferay