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i'll try by breaking down each part for you...

i am assuming you are speaking of this kit...
G.SKILL F2-8500CL5D-4GBPI PC2-8500 4GB 2X2GB DDR2-1066 CL5-5-5-15 240PIN Dual Channel Memory Kit

G.SKILL F2-8500CL5D-4GBPI PC2-8500 4GB 2X2GB DDR2-1066 CL5-5-5-15 240PIN Dual Channel Memory Kit

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G.SKILL = this is the manufacturer name

F2-8500CL5D-4GBPI = this is the model number, like Focus is to a Ford Focus, it also designates specs as you will see shortly

PC2-8500 = the "PC2-xxxx" is a way manufacturers identify the theoretical bandwidth of memory. PC2-8500 has a theoretical bandwidth of 8500MB/s. i think it is actually 8533MB/s but that doesn't matter. this is simply an identifier to group memory together, it really isn't necessary as it is redundant.

4GB = total amount of memory...4 GigaBytes

2X2GB = distribution of total amount of memory above...2x2GB equates to two 2GB modules

DDR2-1066 = this is the important identifier. this is why the PC2-8500 is redundant, they say the same thing. PC2-8500 = DDR2-1066. DDR2-1066 is another made up term to identify memory speed. the 'DDR' portion stands for 'Double Data Rate'. this means that Data is transmitted twice during the clock cycle. so the memory actually runs at 533MHz (which is the memories actual frequency). but because data is transmitted twice with every clock cycle, it is effectively running at DDR2-1066 (533x2 = 1066). by labeling it DDR2-1066, they are identifying that the memory runs at an effective 1066 or DDR2-1066. this is why when memory running at DDR2-1066 shows up in CPU-Z as 533MHz, it is running at full speed. 533MHz = actual frequency, DDR2-1066 = marketing term to make memory look like it is running faster. but 533MHz = DDR2-1066. this is the tough part as there is a bit of back knowledge required to full understand. all you need to really know is DDR2-1066 = PC3-8500. it is the 'speed' of the memory.

CL5-5-5-15 = memory timings. these are the main timings that the memory runs at. standard identification of memory is by these main timings. CL5 = Cas Latency 5. the others aren't really important but you will notice most DDR2 is either 5-5-5-15 or 4-4-4-8 or 3-3-3-8. these are common timing groups. tighter is better as it is faster, less latency, but as the memory frequency increases, timings have to loosen. it is a direct relationship. higher frequency (DDR2-1066/533MHz) = looser timings (5-5-5-15 VS 4-4-4-8 for example). often memory running at DDR2-800/400MHz will have 4-4-4-8 timings, most DDR2-1066/533MHz will have 5-5-5-15 timings.

240PIN = simply the amount of pins the DDR2 DIMM slot uses. all DDR2 is 240pin.

Dual Channel Memory Kit = identifying that the memory kit is a two stick set of memory designed to run in dual-channel. again, pretty standard at this point in time.
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so too answer your question about the two different kits...

G.SKILL F2-8500CL5D-4GBPI PC2-8500 4GB 2X2GB DDR2-1066 CL5-5-5-15 240PIN Dual Channel Memory Kit

G.SKILL F2-8500CL5D-4GBPK PC2-8500 4GB 2X2GB DDR2-1066 CL5-5-5-15 240PIN Dual Channel Memory Kit

...they are identical except for the bolded part. this is part of the model number and designates the heat sinks. that is the only difference. the slightly more expensive kit is the PI heatsinks, the PK are the normal baby blue heat sinks. that is all the difference is. i know that was long but you said you wanted to learn

HTH = hope that helps