When a value cannot be represented in the number of bits allowed, we say that overflow has occurred. Overflow occurs when doing arithmetic operations. Everything represented by a computer is represented by binary sequences. A common non-integer to be represented is a character. The code has some nice properties.
Computers represent real values in a form similar to that of scientific notation.
It too must have a sign. There are standards which define what the representation means so that across computers there will be consistancy. Note that this is not the only way to represent floating point numbers, it is just the IEEE standard way of doing it. Each is just a bunch of bits. This is the sign of the number.