A sign bit is used to indicate the sign of a number. Depending on whether the bit is 1 or 0 the number is considered positive or negative. A char need not be signed because it is used to represent unicode characters. Negative unicode characters just dont make sense. So a char is 16 bit unsigned.
The reason this is interesting is that by not supporting signed numbers a char can contain a larger number than say a short (also 16bit but supports signed numbers).
short can hold a number up to 2^15 - 1 char can hold a number uup to 2^16 - 1
Yes, chars are not meant to hold numbers...but you can assign a number to them.