Doorway pages are typically pages based on templates with the same basic text, but with location names switched out on each page. Door way pages use duplicate content, add no real value, and only exist for the purposes to rank.
After the user lands on the page, they are either encourage to visit another page, or automatically redirected to another page, hence their name.
In the early days of search, door way pages would index pages for specific keywords while sending visitors to a different page. Some doorway pages would also be setup to redirect visitors without their knowledge by using some form of cloaking.
According to Google, "Doorways are sites or pages created to rank highly for specific search queries. They are bad for users because they can lead to multiple similar pages in user search results, where each result ends up taking the user to essentially the same destination. They can also lead users to intermediate pages that are not as useful as the final destination."
No, Google has confirmed this in 2015 when asked to review the Hilton Hotel for having landing pages for each hotel on the main website.
"Those aren't doorways, no. There's nothing deceptive or manipulative that I can see." Eric Kuan, Google
No. If the content is unique, adds value to the user it is not considered a doorway page. Using Hilton Hotels as an example, if a blog about "Top 10 attractions in Toronto" and "Top 10 attractions in Calgary" provides unique information specific to that location that is considered unique and valuable. A general blog about a topic can be posted on a corporate site, but posting the exact same blog across multiple business locations would only service to create duplicate content issues.
Another example could be for a handyman service with two unique blogs about drywall repair. Drywall installation in northern parts of the county compared to southern states can have unique considerations like humidity that the page visitor would want to know more about. In addition, to local climate there may be other considerations, suppliers, practices, or local regulations and tips that are valuable to the reader and answer their questions.