Examples of applications include Microsoft Word, Windows Explorer, and Internet Explorer. These typically are not running when you log in or boot up. You have to open them.
Services are programs most users have never heard of. Examples include Alerter, Logical Disk Manager, and Event Log. Many users are familiar with some services, such as Security Center. Windows services typically are running when you log in or boot up, though a service may be set to automatic, manual, or disabled mode via the Services interface (accessed via the Control Panel).
By default, Windows services do not run as a real user. They run as a virtual user with administrative rights on the machine. If you see "LocalSystem" for the user ID of a rogram, you are looking at a Windows service.
The working directory for a Windows service is typically c:\WINNT, which is the normal designation for the Windows system directory. The default temporary directory is typically c:\WINNT\TEMP. There is no home directory for this virtual user. Consequently, a service can't directly store user-specific data.
This virtual user "LocalSystem" doesn't have access to such resources as network file shares, either. Providing it with such access requires configuring it to run as a domain user with access specified. But doing this gets it out of the pure Windows service role, and saddles it with a password. That brings additional issues and housekeeping baggage.
Why you would want a Service
One big advantage a Windows service has over a Windows application is you don't need a user logged in for the Windows service to run. This resolves many security issues. If a program doesn't really need a user for it to do its thing, then it makes sense to write that program as a Windows service.
Why you would want an app
Windows applications are user-controlled, as opposed to Windows Services which are machine-controlled.
One advantage of having apps is the user can select the app to use, use it, then close it. If you count up the apps on a typical user's machine, you get a long list. How long do you think it would take to boot up if all those apps had to load at startup? Yes, there are some apps that load at startup by default; this is nearly always a bad idea, and you should probably remove all of them from the Windows Startup folder that you can get to from the Start button.
The list of Services is far shorter than the list of apps. Services typically must load at boot-up, though some services load when triggered by a condition or action. If you look at your Services interface, you'll see the Startup Type can be set to manual, automatic, or disabled. Services don't have to load a UI, either. This means they execute quickly, but then it also means you can't interact with them. You do interact with an app. Some apps, such as cViewREMOTE, act as a UI for specific Services.
Another advantage to having an app is you can easily close it, and thus reclaim memory. This is much less of an advantage then it used to be, but it's still an advantage.
A couple of other things:
- When you close an app and re-open it, you also reset it.
- A Service does one thing. An app does many things.
What about the different versions of Windows?
For those who don't know, NT is the kernel behind several variations of Windows. It stands for "New Technology" and was developed by Dave Cutler at Microsoft. Due to internal political problems that drove Dave batty, he quit Microsoft and began racing cars.
Windows 95 (based on DOS) was the beta for the NT4.0 UI. The next releases of Windows (2000, XP, Vista, w7) kept this same OS but just moved menu items around (often with no purpose or sense of logic), changed the graphics, and added security features.