Double quotes work similarly to single quotes, except that they're not as restrictive. 
Whereas the single quotes tell the shell to ignore all enclosed characters, double quotes say to ignore most

$ x=*
$ echo $x # addresses intro lotsaspaces names nu numbers phonebook stat 
$ echo '$x' #$x
$ echo "$x" #*

Q: with or without "
A:
$ twoline="1		# var with 2 line which include a newLine ie "\n"
> 2"
$ echo $twoline		#shell simply removes the embedded newline character, treating it as it would a space or tab: as an argument delimiter. 
			#Then it passes the two arguments to echo to be displayed. echo never gets a chance to see that newline; the shell gets to it first
$ echo "$twoline"	#double quotes tell it to leave any embedded whitespace characters alone. So in this case, the shell passes a single argument to echo