Bicycle tire and rim sizing terminology dates from the 1800's and there was no consistancy as to how the sizes were named.

Several years ago the European Tire and Rim Technical Organization (ETRTO) developed a consistant system that standardized the various sizes and gave them specific dimensions based on the rim's bead seat diameter in mm and tire width in mm. For example an MTB 26" wheel is defined as an ETRTO 559 and a 2.0" MTB tire for that rim is an ETRTO 559 x 50 mm. A 700c road rim is an ETRTO 622 and a 700x23 road tire is ETRTO 622x23. There are LOTS of others.

The International Standards Organization (ISO) adopted the ETRTO standards so you will now also see wheels designated by their ISO size.

Sheldon Brown's web site has the history of this standard and a table showing the various tire and rims and their ISO/ETRTO sizes.