Inkscape is a great, crossplatform, open source tool to draw vector graphics (schemas, etc).
I covered only basics of using Inkscape efficiently. I could not really show how you can prepare your SVG file in order make it ready for animations in a presentation, see an example (use right arrow to go through it).
UPDATE: here is the video, I hope it is understandable despite my English and my accent.
Feedback’s welcome.