http://www.unix.com/man-page/posix/1posix/eval/

eval - construct command by concatenating arguments
eval - it takes a string as its argument, and evaluates it as if you'd typed that string on a command line.                 
eval - utility shall  construct a command by concatenating arguments together, separating each with a <space>. 
       The constructed command shall be read and executed by the shell.

a='abc'
b=a
eval y=\$$b
printf "%s" $y

$(cmd)                  # note: when cmd was braced with $() it just like you enter cmd under prompt $
n=1; $($n)              # this you enter 1 under prompt $, and nothing happen
                        $ n=1; $($n)
                        sh: 1: not found
n=1; eval echo \${$n}   # equivalent echo $1