Back in the day, Cam Studio was the easiest to use and lightest software for screen capture & commenting but I just checked around it looks like it comes with adware bundled, you can still safely install it but you have to make sure you are not tricked into clicking & installing the adware that's bundled. Otherwise I am sure there are many better options out there.
I checked
Alternative.To and this came in as the best alternative:
http://alternativeto.net/software/camtasia-studio/