On every piece of RAM you'll see the timings like: "2-2-2-5".
What is this all about? What should I buy?
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There are a lot of detailed explanations about this on overclocking forums as well as hardware review sites.
A very basic answer is that each number represents the number of cycles it takes before the memory can perform a given internal action. Generally, lower is better, however, it's not always true, as the separate numbers have an effect on each other.
You can think of this as basically a measure of how "quick" the memory is, as opposed to how "fast" it is. It's basically the delay before actions can get started.
Because the numbers measure clock cycles, remember that lower timings don't necessarily mean faster access times. Memory running at 100 MHz with a timing of 2-2-2-5 will have effectively slower timings than memory running at 400 MHz with a timing of 5-5-5-15 for example (even though it takes 5 clocks rather than 2 clocks, it cycles 4x faster).
Overall don't worry about it too much. Frankly, you probably are never going to notice the difference between different timings. The only reason you'd want to worry about it is if you are a benchmarking/overclocking enthusiast.
If you are just looking for a fast PC, and you aren't a big tweaker, you should buy well-reviewed, reliable RAM that comes at a good price. Invest that money you save by not buying low-timing memory in components that will make a bigger speed difference.
Here's the executive summary:
RAM timings are the amount of wait time that the processor spends to get data from memory. Essentially, the processor has to wait a certain amount of time before an access, and then pulls that data at a certain bandwidth (which is a separate measure, like 8.4GB/s). You want the lowest possible latency timings (2-2-2-5 would be awesome) and the highest possible bandwidth.
As far as the w-x-y-z times go, w is CAS latency, x is tRCD, y is tRP, and z is tRAS. These are each different latencies for different operations, and I forget what's tRCD, tRP, and what's what.
If you are an enthusiast and you care, then the most important value is the CAS latency w. Again, low is best. For the next two numbers: neither is very important in my experience. Lastly, tRAS (z) is important to set within a certain tolerance range; essentially unless it has an extreme value it makes little difference.
Overall, these timings are not vital to performance any more. I would not configure them myself (unless I knew what I was doing) because you are more likely to make your system unstable than to improve performance by > 1%.
My recommendation is simply to buy the highest clock rate memory that is available for your given memory generation (DDR2, DDR3, etc.) -- that with the greatest MHz, which maximizes bandwidth. Then let the motherboard choose memory timings.