They are specified by a sign bit, some exponent bits and some fraction bits.  
Official rounding modes for these include unbiased, towards zero, towards infinity and towards negative infinity.  Some examples of these are known as normalized and denormalized numbers. Using these, multiplying infinity by zero results in NaN, or not a number.  The IEEE 754 standard for these specifies them in single- and double-precision forms among others.  They are specified by a sign bit, some exponent bits and some fraction bits.  FTP, name these approximate representations of real numbers used in many computers, so named because the decimal place is not fixed.
Official rounding modes for these include unbiased, towards zero, towards infinity and towards negative infinity.  
floating-point numbers