eCPM is simple, it takes your current impressions (per 1000) and divides that by your clicks and will equal around your earnings. It's a formula to estimate what your earning per 1,000 pageviews.
cpm is just a stadistic of how much are you making per 1000 impressions, this is for comparing advertisers, if you are getting a lot of traffic from social networking sites, by sure your cpm will be lower, cause those visitors dont click your ads. . For example, if i have a cpm of 9$ with adsense, and with adbrite i have a cpm of $6, then, adsense will make more money in my site than adbrite, thats the purpose of the cpm.
Don't be confused, some ad networks do have a CPM, thats is when they pay-per-impression, this is not the case with eCPM and Google Adsense.
eCPM is just a formula to see how much you'll make per 1,000 page impressions. The more "visitors" you have and the more "clicks" will increase your earnings.
No, traffic doesn't nessasary decrease it, the lack of clicks from visitors done, they both come hand in hand.
build your sites for epc, not ctr or ecpm. ecpm becomes important when you are evaluating the total value of the page, as in, you have ads from more than one source on the same page.
It goes like that: MFA website - less traffic, huge number of clicks, people came in, click and that's all. CTR is huge so eCPM is high, because there's lot of clicks. Quality website - more traffic, but this users instead of clicking are reading articles, surfing, - they make more page impressions, so CTR is lower, eCPM also looks lower but it's not. eCPM is medium price for 1000 impressions. But - clicks from quality website give more money then from MFA website, they look like lower, because of huge impressions and low CTR.