* excluding the disabled: http://www.ccdonline.ca/en/socialpolicy/immigration
Original text: 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).
[0] https://en.wikipedia.org/wiki/Visa_policy_of_Canada#Visa_exe... [1] https://travel.state.gov/content/visas/en/visit/visa-waiver-...
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.
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.