Why are Unix applications documented with an number in parenthesis with man?

When I try to get help on a Unix command, I get reference to say tar(1) or lsof(8).

Why is there a number within parenthesis and what does it refer to exactly ?

The manual is divided into chapters and some topics are actually documented twice, which is often the case when there is a C library function or system call that is also available as a user space command or a config file is available with the same name as the command it documents (example: chown is a user space command and a system call and you find chown in section 1 and 2). The number is then used to select the appropriate chapter:

man 1 ls

• Section 1: user commands
• Section 2: system calls
• Section 3: library functions
• Section 4: special files
• Section 5: file formats
• Section 6: games
• Section 7: conventions and miscellany
• Section 8: administration and privileged commands
• Section L: math library functions
• Section N: tcl functions

Yep, what SvW said. You can get the full doc from man itself by typing:

$ man 1 man

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions eg /etc/passwd
   6   Games
   7   Miscellaneous (including macro  packages  and  conven-
       tions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]