A web application is a business management tool of the software variety that runs on a powerful, secure, centralised web server. Because such systems are normally classed as 'mission critical' (in other words a business could lose income or even fail completely without such services), server redundancy and data recovery procedures are standard practice when hosting web applications.
Users of web applications connect to a central web-server via the internet, using nothing more complicated than a browser such as Firefox or Internet Explorer with SSL data encryption (as with secure online banking). This means that the client-organisation only needs to maintain a stable ADSL connection and basic Macs or PCs — the hassle and worry of running and securing application servers and complex databases is removed from the office environment.
What's more, multiple users can instantly connect to the system from anywhere in the world with an internet connection (dial-up or broadband), meaning that staff from businesses spread across several national or global sites can login and work wherever they find themselves — even from home or an internet cafe in the Algarve ...now that's progress!