World of Liferay portlets (Friends Activity, Recent Activity) give you feed of activities about your friends and overall.Here I am explaining how Blogs portlet generates feed for these portlets.
You can follow same steps to implement it, to display your portlet in feeds.
What is web service
Web service is nothing but exposes your software as service for other application over the network. Applications can be written in any programming language and if they support the statandard they can be integrated.It removes the concern of writing custom interfaces and REST/HTTP Post based calls for each integration point.