difference between double inverted coma and single inverted comma

Shell Programming and Scripting
1

Whats the basic difference between double inverted comma and single inverted comma and no comma applied at all?

Eg1 if [ -f "$filename" ]

Eg2 if [ -f $filename ]

iEg3 f [ -f '$filename' ]

2 
'  is a tic or tic-mark or single quote
" is a double quote

quotes affect how the shell treats a variable.

if [ -f $filename ] translates $filename - if it has spaces the shell "thinks" $filename
         is really separate values  with no spaces
if [ -f "$filename" ] translates $filename - if it has spaces the shell "thinks" $filename
         is realy one single value, spaces included
if [ -f '$filename' ] does not translate filename - the shell takes the $ literally, not
         as a shell token meaning return the value of a variable