Starting in Internet Explorer 8, the Web browser was programmed to break its tasks down into separate processes. These processes are what you see in Task Manager when you start the program and begin to use it. This system is called "Loosely-Coupled IE" and it works by opening a new process when you open a new tab or a new Internet Explorer window. This way, if a particular tab becomes unresponsive, you can kill its process in Task Manager and only that one process will close, rather than Internet Explorer and all of the other tabs you have open.
Multiple instances of Internet Explorer may open on your computer when you start the program because of a feature that is designed to enhance its stability. In the past, if one page that you had open in the browser became non-responsive, the entire browser would become non-responsive. Now you can deal with non-responsive pages on a case-by-case basis because of a new Internet Explorer feature.
The more tabs you have open, the more processes you will see. Additionally, if you have a fast computer with a lot of RAM, Internet Explorer will take advantage of this by further segregating tabs into new processes. It might appear that each process is using a lot of memory, but Internet Explorer will not open more processes than your computer can handle. If you think it is using too much memory, you can either close some tabs or kill their processes.
Automatic Tab Recovery
Another reason that you might see several "iexplorer.exe" processes running in your Task Manager when you open the program is because of Internet Explorer's Automatic Tab Recovery function. Internet Explorer is always monitoring itself so that in the event of a crash, it can isolate the troublesome tab and close it. This self-monitoring system requires system resources and those resources take the form of additional instances of Internet Explorer running in the Task Manager.
Internet Explorer Add-Ons
Internet Explore Add-Ons can also show up in Task Manager as an "iexplorer.exe" process because they sometimes open new tabs when you interact with them. If you suspect that an add-on is using a lot of your system resources, you should start Internet Explorer with add-ons disabled and compare the difference in resource usage. To do this, click "Start," type "Internet Explorer" in the search bar and then click "Internet Explorer (No Add-Ons)." You can disable an add-on by opening Internet Explorer in its normal mode, clicking "Tools" and then clicking "Manage Add-Ons." Click the add-on that you want to disable and then click "Disable."