If you intend on traveling to Canada in the near future, then you will require some form of visa to provide travel authorization for entering the country. Similar to the US ESTA, an ETA is an Electronic Travel Authorization available for people visiting this country from visa-exempt countries. It forms part of the border control processes defined by the Canadian Government.
Who can apply for an ETA?

Citizens from a visa-exempt country are able to apply for an ETA, this includes UK, Italy, Australia, New Zealand, and many other countries.
Your application will be assessed and the details you provide will determine whether your ETA is approved (note: criminal convictions or medical problems can affect your outcome).
The ETA is for people that are traveling to Canada for tourism, business and travel purposes, so any other activities such as employment or studying are not permittable. For anyone traveling to Canada for employment, an employment visa must be applied for. On the other hand, there are TN Visa requirements about which you can read more here.
How do I get an ETA?

You can apply for an ETA by completing an online form, in which you will need to provide information such as your name, address, DOB, passport details and some other personal details. You will also need to answer some eligibility questions that include security-based questions including any medical health problems and criminal convictions. For any information related to health visit canadiantravelclinics.ca.
Once you have completed the short online form, you will need to pay the ETA application fee before submitting your application. You can find out more about how to apply for ETA on the website.
Completing the form should only take around ten minutes and usually, it will not take long to get a response confirming whether your application has been approved. If your application gets approved, your passport will be electronically linked to your ETA and when you are at the border control desk when you arrive in the country, the border agents will be able to see you have an ETA.
How long can I stay in Canada with an ETA?

The ETA enables visitors to stay in the country for up to six months at a time. One of the great benefits of an ETA is that it is valid for 5 years, as long as your passport is still valid (must be the one that you applied for the ETA with). During those five years, you are permitted to make numerous journeys to Canada on that same ETA, provided that you do not overstay the six-month limit on any trip. You can find out more about how to apply for ETA on the website.
Entering Canada from the US
If you are visiting the US and then traveling to the country by land or sea, then you will not require an ETA at the border. You will just need your travel documents and ID. You will need an ESTA or alternative visa to enter the US by air, in line with the US border control policy.