Poles, for instance. But even when Canada, as the US, requires visas for most nationalities, it is extremely simple to apply for one (just scan your passport, fill out the online form and pay $7 CAD). On the other hand, applying for a visa to visit the US is a stressful and relatively expensive (~$180 USD) long process with a high chance of getting rejected. This is not new though.
I'll add that, at least here in Brazil, applying for a US visa necessarily involves at least one in-person visit to any US consulate or embassy, in a business day, and they naturally don't exist in all cities. Lines are long, and the visit can take the whole day. So there's this much extra stress and expense.
As others pointed out, Canada recently made mandatory their eTA program which is very similar to US ESTA, so the differences are non-existent anymore.
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The number is huge. For instance the whole EU doesn't need visa for Canada [0] but it does for US (although it's called a waiver program [1], which guarantees nothing).
For friendly Canada all you need is an Electronic Travel Authorization document, whereas the fascist US demands an Electronic System for Travel Authorization document.
The Visa Waiver Program (VWP) allows citizens of 38 countries to enter the US for up to 90 days without a visa.
The Electronic System for Travel Authorization (ESTA) is an online form which asks questions and determines whether someone is covered by the VWP. Using ESTA to determine this in advance is required when arriving in the US by air, but not when crossing a land border from Canada or Mexico.
The "doesn't guarantee anything" bit is true but misleading, since ESTA doesn't provide a visa and doesn't provide an absolute guarantee someone will be admitted under the VWP -- the determination of whether to allow someone to enter is always made at the border.
Couple things that are good to know:
1. VWP/ESTA is not terribly different from some other systems, like Australia's ETA, in using the "you don't need a visa, but you do need to check your eligibility online in advance" approach.
2. Not all EU citizens are eligible for the VWP. Citizens of Bulgaria, Croatia, Cyprus, Poland and Romania are not eligible for VWP. Which is something the EU has been consistently angry about, and recently the European Parliament rattled its sabers a bit about revoking visa-free travel to the EU for US citizens unless the US adds those five countries to the VWP.
> 1. VWP/ESTA is not terribly different from some other systems, like Australia's ETA, in using the "you don't need a visa, but you do need to check your eligibility online in advance" approach.
Relevant to this thread, it's also quite similar to Canada's eTA, which became mandatory for everyone arriving by air, except Canadian and U.S. passport holders, in November 2016.
