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An unsigned integer type can represent only positive numbers and zero.
A signed type can represent both positive and negative number, in a
range spread almost equally on both sides of zero. For instance,
unsigned char holds numbers from 0 to 255 (on most computers),
while signed char holds numbers from -128 to 127. Each of
these types holds 256 different possible values, since they are both 8
bits wide.
Write signed or unsigned before the type keyword to
specify a signed or an unsigned type. However, the integer types
other than char are signed by default; with them, signed
is a no-op.
Plain char may be signed or unsigned; this depends on the
compiler, the machine in use, and its operating system.
In many programs, it makes no difference whether char is
signed. When it does matter, don’t leave it to chance; write
signed char or unsigned char.3
Personal note from Richard Stallman: Eating with hackers at a fish restaurant, I ordered Arctic Char. When my meal arrived, I noted that the chef had not signed it. So I complained, “This char is unsigned—I wanted a signed char!” Or rather, I would have said this if I had thought of it fast enough.