The layout is mostly the same but slightly different between vendors at times... but they generally follow the style that carpetsmoker qouted.
At least when they are so nice to _not_ intermix letters and number-letter combo's such as 1M, N, or X.
Most times you shouldn't need to worry about what section a command is in to find it. Unless there is a program with the same name as a syscall or file format which you need instead, that is...
Example:
man printf -> man page for /usr/bin/printf
man 1 printf -> same thing
man 3 printf -> the standard C function printf() and related routines
man 9 printf -> the printf() function and related routines used within the FreeBSD kernel.
printf() in unix kernels has been around since at least AT&T UNIX Version 6, the standard library function since before there was a standard library (oldest reference I know is a
C tutorial from 1974), and the printf command since 4.3BSD-Reno according to it's manual page.
So most of the worst ambiguities have been with us since the cows came home, went out again, and fell over dead. The ones most people care about however are minor.