The first term is CPM, which is cost per thousand impressions. For example, if you spend $500 on 30,000 impressions, your CPM = $16.66. Most CPMs range from ~$2 to ~$10.

Another term is CPC, which is cost per thousand impressions. This basically means "how much money does each click cost you?" This could range from 10 cents to $20 depending on what you're promoting.

There is also CPL, which cost per lead. A "lead" happens when you collect information (name, email, phone) and can follow up with a potential customer. Your cost per lead is very important.

CPA is cost per acquisition. This really means "how much does it cost to close a customer?" Your CPA should be lower than your revenue gained.

Another term is eCPM, which is cost per thousand ad impressions on a website (i.e. display ads).

The last term is CTR, which is click-through rate. This means "how many people saw/how many people clicked."