EPC is a unique offering of Commission Junction. EPC stands for "average payout per hundred clicks". It is not a guarantee, but it tells you that if you have a site which can promote any specific advertiser as well as their average site does, you can expect to make that much money for each hundred clickthroughs you send them. The EPC is calculated for the latest verifiable 3 month and 7 day periods.
CPM is a standard metric used to calculate the relative cost of advertising. It stands for the "cost per 1000 impressions". (For example, an advertisement costing $5,000, printed once in a magazine with a print run of 100,000, has a CPM of $20.) Pay-for-Performance does not pay this way. You can earn commissions for a clickthrough, for a lead, or for a sale. There are tools on the "Get Links" section of the Publsiher Account Manager to research advertisers on the basis of different criteria.
CTR stands for "clickthrough ratio" --- it is obtained by dividing the number of impressions by the number of clickthroughs. A higher clickthrough ratio means a more effective banner.
Clickthroughs are generated when a person visits your website, and clicks through to the advertiser through one of your links. These appear on "Performance Reports" but not on Transaction Reports, since Commission Junction no longer pays commissions on a clickthrough basis.
A lead is generated when someone goes to an advertiser's website and signs up with their program, fills out a form for more information, downloads freeware, etc. Advertisers can design their lead program in many different ways. Please refer to the program specs for the advertiser when joining their lead program.
An impression is generated when a banner, or text link, displays on a website. This can be a good indicator as to how many times your webpage is visited and also gives you a good idea as to the amount of visitors it takes to generate a transaction through your links.