There are several commercial and free tools available. This post is about one of my favourites. What I particularly like about screencast-o-matic is that it is a webapp - which means it is a "app" that runs from within a browser. You will need Java installed and enabled to run it though (click here to check if you have). This also means the tool will run anywhere where Java is available - and it is on almost all of the major browsers and operating systems. Once you have it, the rest is easy!
Simply go to the website, click on the record button and you are on your way! The easiest way to show you how is with a screencast :) Click on the picture below to see how it's done.
The free version of the tool allows you to upload to Youtube and/ or download the videofile. There's a pro version that let's you edit the video, post to Vimeo/ Google Drive, create and edit captions, etc. The pro version is priced at $15 per year at the time of writing this post.
The reason why I prefer it is simple - it's FREE, easy to use and there's nothing to install (other than Java, of course).
Disclaimer: Screencast-o-matic logo copyright credits to screencast-o-matic.com