Why do some Linux system calls have two man pages? -

example:

http://linux.die.net/man/2/socket
http://linux.die.net/man/7/socket

in way different?

socket(2) provides documentation socket() system call; socket(7) describes how use sockets (in general) on linux.

man man gives overview of sections:

   1   executable programs or shell commands    2   system calls (functions provided kernel)    3   library calls (functions within program libraries)    4   special files (usually found in /dev)    5   file formats , conventions eg /etc/passwd    6   games    7   miscellaneous  (including  macro  packages  ,  conventions), e.g.        man(7), groff(7)    8   system administration commands (usually root)    9   kernel routines [non standard] 

this list more or less (though not quite) "universal" across unix systems, , sections you're interested in of time. wikipedia has more documentation on man sections used in various unix systems.

there many "duplicate" manpages, example crontab(1) describes /sbin/crontab, , crontab(5) describes crontab file format.

you can configure section man uses if don't explicitly add section mansect environment variable; defaults 1 n l 8 3 0 2 5 4 9 6 7 on linux.

for introduction each section, @ man <n> intro (ie. man 7 intro).

bonus tip:
apropos friend:

$ apropos crontab anacrontab (5)       - configuration file anacron crontab (1)          - maintains crontab files individual users crontab (1p)         - schedule periodic background work crontab (5)          - files used schedule execution of programs