There are 3 types of quotes. Each of the quotes bring different meaning and usage.

' a.k.a. single quotes - Everything wrapped in this quote won't be changed (Strong quotes)

" a.k.a. double quotes - Quotes that doesn't expand meta-characters like "*" or "?," but does expand variables and does command substitution (Weaker quotes)

` a.k.a. back quotes - To execute command

Examples of quotes usage (top lines are commands and the output are displayed below the commands):

Example of using back quotes within single quotes. Nothing is changed.

echo 'Today is `date`'
 
Today is `date`

Example of using back quotes within double quotes. The `date` command will be executed:

echo "Today is `date`"
 
Today is Mon May 26 09:42:50 MYT 2008