Once you have the approved eTA, you will be able to travel to Canada and stay there for visits that will not exceed 6 months over the course of 5 years. However, if your passport has expired you will need to apply for a new eTA, the same applies for your eTA, if your travel authorization expires you will need to apply again.
During the 5 years of your eTA validity, you can travel to Canada as many times as you wish as long as your passport is valid and you do not need to extend your stay for more than 6 months.
When you will travel to Canada you will only need to present your passport, do not worry if you do not have the printed version of your eTA approval. Your eTA is directly linked to your passport. While applying make sure that the information you provide on your eTA is exactly like it is shown on your passport. Upon your arrival the officials will scan your passport, unfortunately if the information does not match you will be denied entry to Canada or even denied to board your flight. If you will be denied entry in Canada you will have to apply for a new eTA, pay the fee again and wait for the approval.