I'll attempt to do it in less than a minute.
Essentially, we have two broad groups of people who travel to Canada as tourists, visitors, business people, or foreign students. Some require a visa and some don't. Under the ETA, we will require those who don't require a visa today to obtain an ETA before they come to Canada. U.S. citizens will be exempt in the same way Canadian citizens are exempt from the U.S. ESTA program.
As for the biometrics program, we will be asking for biometrics from a certain subset of nationals from countries that do require a visa today, as I said, based on our assessment and analysis of risks that those populations present to Canada. So of the 150 countries that require a visa for entry into Canada, there will be a subset that will be required to submit biometrics as well. For citizens of the 50 or so countries exempt from the visa, all except American citizens will be subject to the ETA.