A bit is the atomic unit of computation in computers. It can have two values: 0 or 1. The most common way of working with bits is in the form of a byte, which is exactly eight bits.
The sign bit is a bit in a signed number representation that indicates the sign of a number. Although only signed numeric data types have a sign bit, it is invariably located in the most significant bit position, so the term may be used interchangeably with "most significant bit" in some contexts.