Liferay is Portlet technology which follows the JSR 168 and JSR 286 complains.
Liferay is very good open source for portlet technology have very good working group and community.
Portlet technology is similar to servlet technology but portlet is small part of dynamic content in web page.
In normal web application when we send request then server serve the response to the client or browser. Here entire page will be served by one response.
Coming to portlet technology in one web page there may be many portlets and each portlet have its own request and response.
Each portlet can send request and get the response from server then the response belongs to only the portlet which send the request.
We can say portlet is small fragment of dynamic content in the page. Page is aggregation of multiple fragments of dynamic content and each dynamic fragment is managed by one portlet in the web page.
In one page we can accommodate multiple portlet and each portlet is responsible for generate dynamic content for page. Multiple portlet together generate the dynamic content and all the portlet dynamic content aggregation will be generated as one web page.