Your computer's RAM is its short-term memory cache, where programs and data are loaded for quick access. Computer RAM has a size, measured in gigabytes, and a speed, measured in megahertz. Some applications, such as database software that needs to track a very large number of changes to records, benefit substantially from faster RAM. Many home computers will get a modest performance boost from upgrading to faster RAM. Checking your RAM's clock rate speed is the first step towards a RAM upgrade.
RAM is installed on small daughter cards, conventionally called "sticks." On a desktop computer, opening up the case and simply removing a RAM stick and looking for the sticker will tell you how much RAM is on that stick, and what type it is. The type will usually be DDR 2x or DDR 3x, and have a speed listed next to it. Many laptops have a plate covering the RAM held in by a screw. Remove the plate, and remove the RAM to see the sticker, then put it back in place.
Checking the BIOS
Rebooting the computer can let you get to the BIOS (or the UEFI on systems build after roughly 2012). Watch the screen on your computer before it boots up; it may give you a prompt like "Press F2 to enter BIOS." Doing so will let you read the lowest level diagnostics on the system. While specifics will differ slightly by manufacturer, you'll see an option for "System Memory" or "RAM" showing the total amount of RAM on the computer, and its type and speed. Note that it will show you the actual clock speed of the RAM here; for DDR2 RAM, double this to get the effective speed. For DDR3, triple the MHz shown for the effective speed. Always use the effective speed when trying to buy matching RAM for your computer.
For computers that boot straight to the operating system without a chance to access the BIOS, there's a utility called CPU-Z (link in Resources) that will gather most of the information shown in the BIOS screen. All of the RAM related information is on the "Memory" tab. It will show the actual MHz at which your RAM is operating, along with the type (DDR2 or DDR3).
To identify the speed of your RAM on a Mac, click on the Apple icon, go to "About this Mac" and click on the "More Info…" button. You'll get to the System tools, and once there you can click on the "Memory" tab. The dialog that comes up will show you how much RAM you have installed, what speed RAM to get, and how many empty slots are available for upgrades.