ECC = Error Correction and Control...or something to that effect.
It means that it will check to see if while the data went through the ram, if it got corrupt somehow. So basically its for servers where you can not have ANY errors, and must have an uptime of months.
Most motherboards don't even support ECC memory, because it isn't that big of a deal. I couldn't care a less if my Ram made 1 mistake after a week of uptime, odds are it won't effect me.
There is also CPU Cache Level 2 (L2) ECC. That's just like your RAM, but closer to the actual processor....actually, its IN the processor. Meaning that the on-die cache(L2) is a lot faster than the RAM. ECC is nice to have on there also, but not needed.
And of course if the processor can not find the data in the L2 cache, or the RAM, it has to resort to looking on the SLOW hard drive for the information, which is so incredibly slow. That's why people like having a lot of RAM, so that they dont have to spin up those slow HDs.
Does that help? Thirsty for more?