Highlight some lines or part of code

When I present something I often want to mark down some part of code in code block.

It is more understable when in big part of code there is something marked down. I provide context with longer code but focus attention on detail. Eg on differences between two parts of code. 

But there is no way to do it. I could eg click on line numbers to highlight line. I would also like to highlight specific parts of code eg with Ctrl+B.

 Consider also including code compare tool, this is often useful in work to demonstrate something, eg we migrate service so I want to demonstrate differences in DB Schema.