A bit or binary digit is a digit that can take on the value 0 or 1. It is essentially identical to a Boolean value (which can take on values of true or false) with the only difference being the context in which it is typically seen.
A bit is usually considered as a unit of data, and can be combined to create objects with more values. For example, a block of 4 bits has 2^4 or 16 different values, and this represents a hexidecimal digit.