Mr Jones wants to go on holiday to Spain, so he searches on the Internet for a website that sells cheap flight tickets. Mr Jones is lucky enough to find www.dodgy-flights (fictitious) and enters his destination and departure date in an online form. Mr Jones hits the 'submit' button and a few moments later a table is displayed showing all the available flights from all the airlines to his destination in Spain.
In fact, what has actually happened is that a 'web application' has taken Mr Jones request and searched several connected databases for all the available flights which match his request. A web page showing all the options is 'dynamically' created and sent to his browser.
Mr Jones then chooses the cheapest flight and fills in another form. The 'web application' processes his details and credit card, then books the flight for him. A couple of days later, Mr Jones receives his printed tickets in the post.
Mr Jones has just used one (of many) web applications.