The "brain" of a computer is its central processing unit, generally called a CPU. The speed of processors is generally measured in gigahertz, but processors with higher speeds don't necessarily perform better than those with lower speeds.
Hard drives store data used on the computer, including programs and stored documents. Hard drives also store documents, images, videos and sound files. Increasingly, consumers are moving to solid-state drives, which are sometimes called SSDs. These drives are typically more expensive than hard drives of the same size but offer far faster performance and quieter operation.
Once a program is loaded from the hard drive, it is moved to the computer's random-access memory, or RAM. Processors generally don't read information directly off of hard drive, and the delay encountered after launching a program is largely due to its transfer into RAM. When a user works on a document, it's temporarily stored in the RAM. Saving the file causes it to be written to the hard drive or SSD.
To control hardware, computers rely on an operating system. As of 2015, Microsoft Windows is the most popular operating system used on desktops and laptops, although Apple's OS X operating system is popular as well. Web server and supercomputer, on the other hand, are often run on Linux-based operating systems.