The main difference between sing and double quote marks is whether backslash processing of special characters can be used. In double quote the backslash pattern is effective, so if you type in the following command
print "The first line.\nThe second line."
The first line. The second line.
print 'The first line.\nThe second line.'
The first line.\nThe second line.
The back quotes have a absolutely different utility. It is used to enclose system commands for substitution into the command lines. (Note that this utility is not supported on all platforms.) For example in a Linux platform you can add a watermark to your graph like this:
set label "By gnuplot-surprising at `date +%Y-%m-%d`" center\ at screen 0.5,0.5 tc rgb"#cccccc" plot sin(x)
The watermarked picture looks like this
|Back quotes in gnuplot --- substitution|