CTR is click through ratio, the higher the CTR, higher the earnings. CTR will be higher for better placed ads and lower for bad placement. So, obviously more visitors clicking on ads = higher CTR = higher adsense $$
CTR = Click Throught Ratio if you got 1000 impression 2 people click your ads then your CTR Will be 0.2%
People often say forums usually have a low CTR, I don't know I haven't got one. More traffic is usually the answer to more earnings, and for most sites increasing traffic is a better way to increase earnings than worrying about CTR. I'd try making the ads the same colour as the rest of the page, see if that helps. I'd also test having the skyscraper on the right instead of the left.
It depends on the layout of the site. If you don't already have a layout, try placing the ads in visible areas.