Real is the head used for real (floating-point) numbers.
_Real can be used to stand for a real number in a pattern. You can enter a floating-point number of any length. You can enter a number in scientific notation by using the form mantissa*^exponent. You can enter a floating-point number in base b using b^^digits. The base must be less than 36. The letters a-z or A-Z are used in sequence to stand for digits 10 through 35.
Real is also used to indicate an approximate real number in Read. See The Mathematica Book on the web: Section 2.11.8 and Section 3.1.1. See also: RealDigits, BaseForm, Number, Reals.