Who requires an eTA?
Section 190 of the Regulations describes individuals who are exempt from the requirement to obtain a TRV. If travelling to Canada by air, these individuals require an eTA unless they fit one of the limited exemptions.
An application for an eTA is done online and costs $7 CAD. All family members flying into Canada must hold an eTA irrespective of age or dependency.
Individuals applying for an eTA need to provide information on their application form regarding their passport or travel document; personal details; occupation; previous travel; responses to background questions (to assess for health, criminality and immigration-related concerns); and contact information. No documents are required with an eTA application. However, additional documents may be required manually during processing.
As per section 12.05 of the Regulations, an eTA is valid for 5 years or until the applicant’s passport expires, whichever occurs first.
As per IRCC’s Program Delivery Instructions, no counterfoil will be provided to an applicant upon approval of an eTA (unlike a TRV). “Therefore, there is no official physical proof of the presence or validity of an eTA. Air carriers will therefore use the CBSA’s IAPI [Interactive Advance Passenger Information] system to confirm that a CIC authorization to travel (visa or eTA) is linked to the traveller’s passport”.
Who is eTA Exempt?
Subsections 7.1(2) and 7.1(3) of the Regulations describe the individuals who are exempt from the eTA requirement. These include: TRV and Temporary Resident Permit holders; US citizens; citizens of France who are residents of St. Pierre and Miquelon; diplomatic passport holders; visitors, students or workers who seek to re-enter Canada after solely visiting the US and returning to Canada before the expiry of their current IRCC document; some airline crew members; transit passengers under specific circumstances; and a few others.
How are eTAs processed?
IRCC advises in its Program Delivery Instructions that if no adverse information is found on an applicant, the Global Case Management System (GCMS) will automatically notify the applicant by email that their eTA has been approved. As a result, most eTAs are expected to be approved within minutes of applying.
When eTA applications cannot be automatically approved, they are referred by the system for manual review by an officer. Officers may request additional documents from the applicant and a security screening (if applicable). If documents are required from the applicant, they will be directed to create a MyCIC account, to which they will link their eTA application.
If a manual review is necessary, an applicant can expect an email from IRCC within 72 hours advising them of the next steps. As the system is new, IRCC has not yet offered definitive processing times. 
Issuance of eTAs when Applying for Work/Study Permits
If a national from a country that requires an eTA is applying for a Work or Study Permit, they will be issued an eTA as part of their temporary residence application. Does this then mean that a national requiring an eTA can no longer apply for a Work Permit at the port of entry? Not necessarily.
To begin, we have the situation of applicants from visa exempt countries that will be arriving in Canada by land or sea. Since an eTA is not required to enter Canada through such ports, these applicants will still be able to process the Work Permit at the port of entry and there is no need for them to apply for an eTA beforehand.
There is also nothing in the Act, the Regulations or the government’s policy that provides that an applicant cannot simply apply for an eTA prior to travel by air and then make an application for a Work Permit at the airport.
A situation where this may be beneficial would be when the client needs to be working in Canada sooner than the time that is taking the visa office to process a Work Permit. Since eTAs are processed quickly, the client may want to apply for this first and then process the Work Permit application at the airport. Bear in mind, however, that if there are inadmissibility issues, the eTA will not be processed right away.
As a general rule, we would advise that if the client can afford to wait until the Work Permit is processed abroad, then perhaps this is a better route which provides more certainty prior to travelling. However, we do not see how the eTA process has restricted the right for individuals who are visa exempt to apply for a Work Permit at the port of entry pursuant to subsection 198(1) of the Regulations. If an eTA prevented a client from applying for a Work Permit at the port of entry, it would contravene the sections of the Regulations which specifically permit it.
Source: IMEDA Immigration Education