What is a Bottle Neck
In regards to computer hardware, and playing games or using software, you will sometimes hear that the computer goes slow because of a bottle neck. A bottle neck is essentially where the computer is slowing down at because its receiving more data than it can handle. Like the term, "a chain is only as strong as its weakest link", likewise, "a computer is only as fast as its slowest component".
How to spot a Bottle Neck
When you find out what you're doing that causes the computer to slow down, you've discovered or are very close to discovering where there is a bottle neck on your computer.
Video Card
If you play a game and it runs slower the higher the graphics, your video card isn't fast enough so the rest of your computer is just waiting on it. Your processor sometimes will pickup the slack, which causes your processor to become a bottle neck.
Processor
When a computer has alot of calculations to do, but it can't handle them- programs tend to run really slow. The computer usually saves those calculations in it's memory for future/immediate reference.
Memory (Ram)
As you open programs, they pull their information from the hard drive and store it in memory. Memory was invented because hard drives were too slow and so they needed a place they could quickly access data from. When you use too much memory though, your computer stores the information on your hard drive.
Hard Drive
This is typically what slows down the rest of the computer. Adding memory helps programs that are already running to run a bit faster. However, when the hard drive slows down, the rest of the computer sits there waiting on it. If the computer seems slower over time, and you've done your basic maintenance, then your hard drive is slowing you down.
Upgrading doesn't always speed everything up
Upgrading certain components will increase speed in certain areas of the computer. Lets walk through the process of how your components are used.
When you open up a graphic intensive game, your computer reads from the hard drive and loads the game into memory. The game uses the processor to make sure that the game is loaded right and it starts some intro movie. When you load a map on the game, again, it reads it from the hard drive, puts it in memory, and then uses the processor to understand the coding of the map, and then it uses the video card to display that information on your screen.
If you increase your hard drive speed, you'll notice that loading the game, and loading game levels will be faster- but it won't allow you to have better graphics and the speed of the actually game play will most likely be the same. If you increase the memory, then you decrease how often the game has to look back at the hard drive for information. Increasing the processor speed usually won't increase load time (unless you already have a really fast hard drive), it would increase the game play, but not necessarily the graphics. If you got a better graphics card then you would have better graphics, but the load time of the game, and the ingame play would usually be the same (unless your processor was picking up slack from your older video card and is now running how it should).