From the TeXbook, page 286, last but one bullet point:
⟨letter⟩, ⟨otherchar⟩, \char⟨8-bit number⟩, ⟨chardef token⟩,
The most common commands of all are the character commands that tell
TeX to append a character to the current horizontal
list, using the current font.
In unrestricted horizontal mode, a