The ANSI layout is the American keyboard layout, primarily used in the United States and Canada. ANSI is the American National Standards Institution. This layout has 109 keys and a vertically short but wide Enter key (more specifically, the Return key on the alphanumeric region). The key for the backslash (\) and pipe (|) characters is between the Return key and the Backspace key.
The ISO layout is the international layout, primarily used in Europe. ISO stands for International Standards Organization. This layout has 110 keys. Its Return key is tall, occupying the vertical space of 2 keys, but it's also horizontally narrower, so the key that used to be above it goes to left. The left Shift key is shorter, making space for the 110th key. The backslash character is typically on the right side, next to the Return key, while the pipe character is on the left side, next to the Shift key.
Keyboards of many languages are based upon on the ISO layout, not upon the ANSI layout. This means that the ANSI layout may be the "normal" layout in the US, but elsewhere the ISO layout is the "normal" layout.
Can I Use an ISO Keyboard if I'm American?
You can use any keyboard in any layout in a typical computer. When you press a key, an electrical signal is sent to the CPU, but before it reaches any application, the operating system translates those physical hardware signals to their appropriate logical software equivalent.
You can customize how the operating system does this by setting the LOGICAL keyboard layout in your operation system. This means you can even try a different keyboard layout right now by telling Windows to interpret the signals as if they were coming from a keyboard in a different PHYSICAL keyboard layout.
Quotes
North American computers traditionally use the ANSI keyboard, while Europeans use the ISO keyboard. These two layouts are almost identical with the exception of several keys. The ANSI keyboard is set in INCITS 154-1988 [S2009] – Office Machines and Supplies – Alphanumeric Machines – Keyboard Arrangement and has 109 keys. As established in ISO/IEC 9995-2:2009 – Information technology – Keyboard layouts for text and office systems – Part 2: Alphanumeric section, the ISO keyboard has 110 keys, along with a smaller left shift key to accommodate the extra key. These two layouts are incredibly similar and still maintain interoperability between the two.
There is also a third type of standardized QWERTY layout, which is covered in JIS X 6002:1980 – Keyboard layout for information processing using the JIS 7 bit coded character set. This allows for simple typing for using both the Latin alphabet and Japanese systems of writing. It bears more distinctions from ANSI and ISO than the two layouts have with each other, but it is still very similar to them both.
https://blog.ansi.org/standardization-of-keyboard-layouts/ (accessed 2024-12-04)
Observations
Fun Fact: the keyboard I use is for Brazilian Portuguese, so it's neither ISO nor ANSI, its standard is by the ABNT (Associação Brasileira de Normas Técnicas, "Brazilian Association of Technical Norms").
Before writing this article I had no idea Americans had such weird keyboards! I never noticed it, but I always press the Return key by pressing it on the upper part, never on the lower part, so if I tried to use an ANSI keyboard I probably would struggle with it a lot!