A monospaced font, also called a fixed-width or non-proportional font, is a font whose letters each occupy the same amount of space. This contrasts to variable-width fonts, where the letters differ in size to one another.
The first monospaced typefaces were designed for typewriters, which could only move the same distance forward with each letter typed. This also meant that monospaced fonts need not be typeset like variable width fonts and were, arguably, easier to deal with. Read more about monospaced fonts on Wikipedia.
