This article is from the Golf FAQ, by marcelo@nntpserver.Princeton.EDU (Marcelo A. Gallardo) with numerous contributions by others.
For the most part, the number "represents" the loft of a club. The
lower the number, the lower the loft and the longer the club. The lower
loft and longer club will result in greater distance with a loss of
accuracy. This also equates to lower numbered clubs being more
difficult to hit.