CPU-Z is a free utility that provides useful information on many aspects of your PCs hardware.
If you've just bought a new system, for instance, then CPU-Z can help you check that you've got exactly what you're expecting. Just click the appropriate tab and it'll display the CPU type and speed, motherboard, RAM amount and speed, graphics card model, and more.
Each section provides many extra details that are particularly useful when you're upgrading. If you want to add more memory, say, then it's probably a good idea to match the speed and type of RAM you have already. You could open the case to have a look, but it's easier to run CPU-Z, and check the Memory and SPD tabs. They'll provide the RAM type, speed, memory timings, manufacturer, even the part number, everything you need to know.
And if you're planning to overclock your CPU or graphics card then you should start by noting your current core speeds, multiplier, bus speed and more. This is probably available in the BIOS, or maybe the Control Panel Display Settings applet, but once again it's much simpler to use CPU-Z. The program supports all the latest hardware, and the answers you need are usually just a click away.