A guy told me that it's very easy to make money by CTA(click through adwords),but I want to know if CTA is against TOS? Thanks
No, as long as you adhere to both the TOS of AdWords and AdSense, arbitrage is completely on the level.
Well, it is and it isn't. Its not as easy as throwing up soem AdWords ads and raking in the cash. You need to do a LOT of research before getting started to make decent scratch and even then it can be risky relying on AdSense for all of your income (ie if you violate the TOS with your blending techniques and get banned, AdWords still gets its money) Be careful but as I said, simply advertising AdSense sites on AdWords is completely fine with the TOS.
You create AdWords ads which advertise your site hoping when people come to that site they click on your AdSense ads. The part that requires research is that in order to be succesful you need to pay less to get that visitor then you make with the AdSense click. It is made more difficult by the fact that not every person clicks and even if they do, you are only getting a commision on that click, not the full amount the advertiser paid.