Back in the early 1900s, a man was given the task to come up with a marketable way to utilize the left over yeast being dumped by breweries. Concentrating the clear liquid extract and blending with salt, celery and onion extract formed a sticky black paste. Marketing this unattractive paste as a must-have was not that easy. After much effort to give out coupons, declaring Vegemite as a rich source for vitamin B and giving them out as army rations, by the end of the 1940s Vegemite had become a household name in Australia. For more details about the history of Vegemite, see this article.
So what does it taste like? Well, watch this video where American kids see and taste Vegemite for the first time in their life. They will describe the taste for you!
For recipes, you can refer to the above diagram. I would highly recommend you stick to the above recipe for beginners, it is a matter of your survival. Believe me.