I was denied entry to US recently because of improper documents. My passport was stamped with entry withdrawn. I have a multiple entry visa also for Canada. Will it affect my travel plans to Canada? Will it affect my visa applications in other countries? I travel a lot because of business.
There are two kinds of ways that you get to not enter the US at a land border. The most serious is "denied entry". This means they officially refused you entry. It goes on your record in the US and you have to answer "yes " to any questions about being denied entry to a country. It will seriously affect your ability to enter the US and other countries.
The much lesser one is "application for entry withdrawn". This usually applies when the reason preventing your entry is more minor, for example not having a document you need. The technical line is that they allowed you to withdraw your application to enter the US, meaning that they did not have to deny you entry. While there is a record of this interaction, it will probably not affect your ability to enter the US or other countries (apart from maybe some additional questioning). You can truthfully answer "no" to questions about whether you have been denied entry or refused a visa.
It sounds like you have the second case, but your describing yourself as "denied entry" makes me uncertain. They should have explained this when they stamped your passport. If you have other documents from the officer, check that they tie up with what I said here.