Disclaimer: this information is about entering Canada in normal situations; there are currently a number of restrictions related to the current pandemic (quarantine, test requirements), in addition to the below. These may change quickly and without warning. Visit CBSA, and Transport Canada, websites for the latest up-to-date info.
In ordinary circumstances, Canada welcomes millions of visitors every year, for business, tourism, or transit to another destination. Just as with permanent immigration, however, there are strict IRCC guidelines that have to be followed from an application, to actual entry and time spent in Canada.
Some travelers do not need a visa to enter Canada, as their countries are permitted to use electronic Travel Authorization (eTA). These are non-paper, web-based applications, used by routine and regular travelers who need to enter or leave Canada repeatedly for air travel. Depending on the expiration date of the travelers passport, they may last up to five years before renewal. Note that even these trusted travelers have to satisfy CBSA requirements to be allowed entry (for more details, see this blog post.)
Ordinarily, eTAs and visas are granted for a maximum 6 month stay in Canada at a time, though a longer request may be granted if there is no reason to deny it (and with special exceptions such as super visas, which are a topic for a different post). Passport expiration is an important limitation; IRCC will not grant stays longer than a passport remains valid, which is why it is important to check the date and renew it if necessary.
When examining an application, IRCC examines for admissibility, the purpose of travel, and ties to the home country. (The below are just broad guidelines: speak to a lawyer before making an application!)