For the ones that still find these terms a little confusing, in this article I'll try to make some light in this matter and to provide easy to understand examples which are representative for the three types of applications: web, desktop and mobile.
Desktop applications are the ones which you can access from your desktop and are installed on your PC; Web apps are accessed over a web browser / network and you need an Internet / Intranet connection to access them; While mobile applications are applications developed for smartphones and tablets. Pay attention that there are also web applications for mobile devices, called mobile web application. The difference between them and mobile applications is that the former run on a web browser / microbrowser, such as: Android and BlackBerry for mobile, or Firefox for mobile.
With an increasing number of households that have Internet access, (only in UK, in 2010, 73% of the households had an Internet connection), the web applications market has become a potential threat for its desktop counterpart. Although some are already saying that the web apps will soon replace the desktop apps, I'm quite sure that this will not happen in the near future, and the truth lies somewhere in the middle.
Due to the fact that we become more and more mobile thanks to our smartphones, netbooks, tablets, e-readers etc, we require permanent access to Internet. Of course that this triggers an increase in the number of web apps, but this does not necessarily mean that the web will soon exclude the desktop. As many of you, I do have a netbook and a smartphone that I use in my work, but when I go home I still have many reasons to turn on my personal computer: either for playing, for watching videos on my extra-large LCD , Or for simply browsing the net.
Indeed web apps and mobile apps offer mobility, but desktop apps are there on your PC even if you loss the Internet connection. On the other hand, once you bought a desktop app is there to stay, whereas web and mobile apps may require from time to time additional subscription fees.
In terms of security, the online tend to be less secure. Let's face it, you are more likely to get a virus while you are browsing the net then using an office applications. For companies this is quite a big issue, so it's only normal that they will want to protect their work.
Also, "speed" plays an important role in this dispute. For sure the apps on your desktop will start immediately when accessed, while the web apps may take a while. If in the first case you are the only one who is running the application, in the latter you are sharing the same application with many other users, so the speed rate for sure will have to suffer.
If until now I've talked mostly about desktop and web, let's see what mobile apps can do. In general mobile apps help users to access the Internet on their devices, but not only. There are mobile apps which are pre-installed on your smartphone, and there are others which you can download via the Apple store, Android store etc. A good example of a mobile application is "Angry Birds".
Such apps need to be supported by an operating system for mobiles, such as: iOS, Windows Mobile or Palm webOS etc. This is why an application is native to only one mobile OS; This means that an app that runs on iPhone will run on iOS device and will not be available to Android users. On the other hand, the mobile web app because it runs inside a browser, it can be accessed via Internet by different types of smatphone users: BlackBerry, iPhone, Android etc.
For sure the number of mobile apps will increase, but this does not mean that we need to pick a side right now. The future is not made exclusively of web / mobile / desktop applications. Until the merging technologies will say otherwise, we can employ the three types of apps to serve or daily needs.