x < y yields True if x is determined to be less than y.
< < yields True if the form a strictly increasing sequence.
Less gives True or False when its arguments are real numbers.
Less does some simplification when its arguments are not numbers.
For exact numeric quantities, Less internally uses numerical approximations to establish numerical ordering. This process can be affected by the setting of the global variable $MaxExtraPrecision.
See The Mathematica Book: Section 1.5.6.
See also: LessEqual, Greater, Positive, Element.