While Target Disk mode requires you to restart the Mac, Target Display mode can be invoked from within OS X.
To do this, first check the model of your iMac to ensure it was made in 2011 or later, and then simply press Command-F2. Note that on most Macs, the F-keys are assigned to a system function by default, so you will either have to revert this in the Keyboard system preferences, or hold the "Fn" key in addition to the Command key before tapping F2.
When you press this hotkey, the system will remain running, but the display will now be accessible through the Thunderbolt connection of the iMac.
You might wonder why this is a feature, since it bypasses a perfectly good iMac computer to use it as an external monitor. While it is true using the iMac itself may be preferred in many cases, in others you might have a laptop (and especially one with a small monitor) configured with software to perform a specific task, and you might need to view this on a larger display. If none are around, then you can use the iMac for this purpose.
The only thing you will need for this is a Thunderbolt cable.
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