A computer's CPU, the Central Processing Unit, is the thing that makes your computer a computer. Without one, the computer you're using right now would be nothing more than a collection of plastic and metal parts. While knowing all the details about the CPU is not essential to using the computer, it doesn't hurt to know a bit about what it is and how it works, so that you can get the best user experience possible.
Basically, the CPU of your computer is the computer itself. The CPU -- also referred to as a "processor" -- contains thousands of transistors that alternately block and transmit electricity. Your CPU receives electrical signals and then changes the signals to something else to perform the tasks you need to do on your computer.
"Capacity" comes into the equation when you start looking at how many "bits" of information the CPU can process in one cycle. The more bits the CPU can process, the faster your computer is, and the more processes it handles at one time. As of 2012, most new computers are made with a 64-bit chip. Another factor is the speed at which your computer's microchip works; the faster the processor, the more speed and processes you'll be able to perform at once. Also important is the number of cores; the more cores your computer has, the more processes it is able to handle at once.
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Checking Capacity: Mac
Since maxing out the CPU's capacity means you may not be able to run multiple programs at the same time or do other multitasking, it doesn't hurt to monitor your CPU capacity. Doing so is simple on a Mac computer. Open the Activity Monitor from the Applications folder and then click on the "CPU" tab. From there you'll be able to see what percentage of the CPU each of your open applications is using; if you find that one application is hogging the CPU and you don't need it to be open, close that application. For some applications, this may allow other programs to run faster.
Checking Capacity: PC
For PC users, monitoring your CPU for capacity overload or other benchmarks is not typically available as a built-in feature. However, a number of third-party applications allow you to do this task. Among the tools available for download for PCs are ipMonitor, NVIDIA System Monitor and CPUID's PC Wizard.