Stepping is a designation used by Intel and AMD (or any semiconductor manufacturer[citation needed]) to identify how much the design of a microprocessor has advanced from the original design. The stepping is identified by a combination of a letter and a number.
Typically, the first version of a microprocessor comes out with stepping A0. As design improvements occur, later versions are identified by changes in the letter and number. Changes in the number (for example, A3) indicate minor design changes, while changes to both the letter and number are made following more extensive changes (for example, B2). Stepping allows consumers and servicers to identify a microprocessor's version.
http://en.wikipedia.org/wiki/Stepping_(version_numbers)