Quote types.

  
" - double quotes.
  Quotes blanks (spaces and tabs)
    greet="hey there"
    echo $greet

  Allows variable expansion. 
    biggreet="$greet everyone"
    echo "$biggreet"

  Suppresses filename wild cards.
    echo * # shows all the names in current directory
    echo "*"  # shows a *

    Useful when passing filename wild card as an argument.
    find . -name "fn*"

  Quotes single quotes.
    echo "She's mine"
    She's mine

  Allows command substitution, ` grave  and $()  (see below)
    list="These are my a files : $(ls a*)" 
    These are my a files : assn02 assn02l1 assn1.txt
  

  
' - single quotes.
  Quotes double quotes.
    echo 'Got "The Best" deal' 
    Got "The Best" deal

  Suppresses variable expansion. 
    echo '$biggreet'
    $biggreet

  Suppresses filename wild cards.

  Suppresses the command substitutions ` and $()
    list='These are my a files : $(ls a*)' 
    These are my a files : $(ls a*)


  
\ - escape quote or back-slash.
  Escapes single characters. Useful when you need a single single or double
    quote character or you need quotes inside quotes.

    echo She\'s mine
    She's mine

    echo "I'm \"Here\""
    I'm "Here"
  

  
[ctrl]v - ctrl quote.
  Escapes other control characters and some other characters such as tab.
    [ctrl]v[ctrl]m - control escapes carriage feed.
 

  Command substitution
  
` - grave quote. Also available in c-shell and others.
  Executes command inside of graves and replaces command with output
    in current line.
  Most command-line evaluation will be applied to command sequence quoted.
  However, grave substitution is deprecated in bash and may disappear 
    in further versions.

  num1=7;num2=9;echo "expr $num1 + $num2 = `expr $num1 + $num2`"
  
  
$() - command substitution - bash specific.
  Executes command inside $( ).
  $() has the advantage of being able to be nested.
  
  echo ls > cmd1 # put command in file.
  echo cmd1 > cmd2  
  $(cat $(cat cmd2))
  var=$($(cat $(cat cmd2)))
  echo $var
  
 
  
  Note : While you can use variable references in the sequence, they are 
    generally evaluated first so usually useful on for the inner-most $()
 
 
More detailed discussion in "quotes" module.

Also :
  http://www.tldp.org/LDP/abs/html/commandsub.html