BaseForm[expr, n] prints with the numbers in expr given in base n.
The maximum allowed base is 36. For bases larger than 10, additional digits are chosen from the letters a-z.
You can enter a number in an arbitrary base using base^^digits.
When a number in an arbitrary base is given in scientific notation, the exponent is still given in base 10.
You can mix BaseForm with NumberForm and related functions.
BaseForm acts as a "wrapper", which affects printing, but not evaluation.
See The Mathematica Book: Section 2.8.7 and Section 3.1.3.
See also: IntegerDigits, RealDigits.