Difference between wait and sleep

wait waits for a process to finish; sleep sleeps for a certain amount of seconds.

wait is a BASH built-in command. From man bash:

    wait [n ...]        Wait  for each specified process and return its termination sta-        tus.  Each n may be a process ID or a job  specification;  if  a        job  spec  is  given,  all  processes in that job's pipeline are        waited for.  If n is not given, all currently active child  pro-        cesses  are  waited  for,  and  the return status is zero.  If n        specifies a non-existent process or job, the  return  status  is        127.   Otherwise,  the  return  status is the exit status of the        last process or job waited for.

sleep is not a shell built-in command. It is a utility that delays for a specified amount of time.

The sleep command may support waiting in various units of time. GNU coreutils 8.4 man sleep says:

    SYNOPSIS        sleep NUMBER[SUFFIX]...    DESCRIPTION        Pause for NUMBER seconds.  SUFFIX may be ‘s’ for seconds (the default),        ‘m’ for minutes, ‘h’ for hours or ‘d’ for days.  Unlike most  implemen-        tations  that require NUMBER be an integer, here NUMBER may be an arbi-        trary floating point number.  Given two or more  arguments,  pause  for        the amount of time specified by the sum of their values.

sleep just delays the shell for the given amount of seconds.

wait makes the shell wait for the given job. e.g.:

workhard &[1] 27408workharder &[2] 27409wait %1 %2

delays the shell until both of the subprocesses have finished