CTR = Click-through rate : is a way of measuring the success of an online advertising campaign. A CTR is obtained by dividing the number of users who clicked on an ad on a web page by the number of times the ad was delivered (impressions).
For example, if a banner ad was delivered 100 times (impressions delivered) and one person clicked on it (clicks recorded), then the resulting CTR would be 1 percent.
RPM = Revenue Per 1,000 Impressions : RPM is similar to CPM, but measures the revenue from 1,000 ads impressions instead of the cost of the ads. Therefore, while CPM is typically measured by advertisers, RPM is monitored by publishers.
For example, a publisher has a website that gets 5,000 page views each day. If the advertisements on the website generate a total of $25.00 of daily revenue, the website has an RPM of $5.00 ($25 / 5). Web publishers use RPM as a way of measuring how effective advertisements are at generating revenue.
Source: Wikipedia