In the latest MacBook Pro systems from Apple (including some before the aluminum "Unibody" design), many models contain two GPUs: a discrete GPU and an "onboard GPU." The discrete GPU is a dedicated high-performance graphics chip for processing graphics data without impeding on the system's RAM and other resources. On the other hand the onboard GPU is an embedded graphics option that shares system RAM and other resources. As a result it is a slower graphics processing option, but when used the system can turn off the discrete GPU and thereby save more battery life.
To specify which GPU to use, Apple has some settings in the Energy Saver system preferences, though these may differ depending on the system you have.
On 2009 MacBook Pro systems there is the option to choose "Better battery life" or "Higher performance" in the Energy Saver settings, which tell the system to use either the onboard or discrete GPUs, respectively. Apple does not support auto-switching on these systems, so to change the GPU with the Energy Saver preferences you will have to log out and log back in.
On 2010 MacBook systems and later, Apple includes support for auto-switching, and as a result the energy saver options have changed to a selection for turning on auto-sense, or to have the discrete GPU on at all times. Unfortunately Apple's options for this are to either have auto-sensing on, or when it is off then only the discrete GPU is used, so there is no way to force the system to use the onboard GPU.
While Apple's settings may be convenient for most people, there are times when you may want more control over which GPU you are using. Additionally, even though Apple does not support auto-switching on 2009 and earlier multi-GPU MacBook systems, technically this can be done to prevent the need for logging out and back in every time you want to change your GPU settings.
To overcome these limitations, if you have a multi-GPU MacBook Pro, try installing the utility "gfxCardStatus", which is a small free tool that allows you to specifiy which GPU you would like to use. The tool allows for several key features that Apple does not support out of the box, including the following:
Auto-switching on 2009 MacBook Pro systems and earlier
Technically the system does not auto-switch GPUs, but you can select which GPU to switch to without having to log out and back in on these systems.
Use either the onboard or discrete GPUs
On Apple's latest systems there is no way to force the system to solely use the onboard GPU, and gfxCardStatus allows this option.
Power source-based switching
Even with auto-switching systems, Apple does not have an option to change the GPU depending on whether you are using your battery. gfxCardStatus allows for specifying which GPU to use when on battery, and which to use when using your charger.
Overall, if you want to save battery life on your systems, use the Energy Saver system preferences to turn off the discrete GPU, but if you want even finer detail on how the GPUs are handled, then gfxCardStatus is an absolute blessing.