CylonRaider has explained it quite well. Just to elaborate a little, CPU is your Processor, and the main force of your PC. For what you want, I doubt you'd even need any of the ''i'' processors. But 'i3, i5, i7 are the newest and fastest processors around, and as CylonRaider says, 7 being the best of that bunch. Next down is Core Duo/DualCore hich are probably enough for you, but I still recommend something more up to date for longevity.
RAM is best described as the memory the computer uses while it stays on. Some people wonder why their computer ran a function much faster today than it did yesterday, and finally figured it out by understanding RAM. If the computer was on for a few hours yesterday before they ran that function, then it had used a lot of RAM previously. While today, they ran the function after turning the computer on for the first time. For what you need, 3GB of RAM would be more than enough, but maybe aim for 4 minimum, again thinking ahead.
Hard drive is storage. Simple as that really.
After that, you could look into graphics cards? An onboard standard graphics card will do what you need, but having a ''dedicated graphics card'' will mean that the graphics run off separate memory to the rest of the computer, where with an onboard card, it runs of the same memory as the rest of the computer. Dedicated graphics cards help computers that might want to run powerful software, such as Design software, Adobe Photoshop or After Effects for example, AutoCAD design software, music recording software, such as cubase, and games. Again, thinking longterm, if you can afford a dedicated card, consider it.
