ANSI and ISO layouts
Also called ANSI · ISO · keyboard layout
ANSI and ISO are the two common physical keyboard shapes. ANSI has a wide one-row Enter key and a long left Shift while ISO has a tall Enter key and an extra key beside a shorter left Shift.
Physical shape and software layout are separate. A UK ISO keyboard set to a US layout types the wrong symbols on several keys without anything being faulty.
In practice
The physical layout decides the shape of Enter, the size of left Shift and whether there is an extra key next to it. Buying keycaps, a replacement keyboard or a laptop from another country means checking which one you have. Mixing a physical layout with the wrong software layout produces symbols in odd places, such as the backslash and the hash key swapping.
How to check
Look at the Enter key. One row tall and wide means ANSI. Two rows tall and shaped like an upside down L means ISO. Then press the keys around Enter and left Shift in the keyboard test. Each one should light where you pressed it. If the characters come out wrong, change the layout setting rather than the keyboard.