Why are there multiple Chrome instances running even though I only have one window (only one tab) open?

You can read the details here: http://blog.chromium.org/2008/09/multi-process-architecture.html

Google Chrome takes advantage of these properties and puts web apps and plug-ins in separate processes from the browser itself. This means that a rendering engine crash in one web app won't affect the browser or other web apps. It means the OS can run web apps in parallel to increase their responsiveness, and it means the browser itself won't lock up if a particular web app or plug-in stops responding. It also means we can run the rendering engine processes in a restrictive sandbox that helps limit the damage if an exploit does occur.

Basically, each tab has one process unless the tabs are from the same domain. The renderer has a process for itself. Each plug-in will have one and so will each extension that is active.

These separate processes are called SANDBOXING. This allows A process to crash, but keeps other processes from crashing a system (or Chrome)

You can see which process does what on:

Wrench Menu->View Background Pages

Which looks like this:

Don't forget to read the Chrome introduction comic that covers this among other design decisions:

Google Chrome use separate process for each tab. Using interprocess communication you can manage multiple processes as one application.

Using multiple process can help an applications to be more robust and better use system resources.

See:

• Post on the Chromium blog - for the reasons google use multiple process in Chrome.
• This stackoverflow answer - for a better detailed programmer wise explanation.
• The IPC value on Wikipedia - for more general description of inter-process communication.