There have been a few threads posted about these, but I don't see a lot of information about them here and I'd like to learn more. It appears that most countries that participated in the First World War provided postcards that soldiers could mail from the field, usually for free. I've come across examples from France (courtesy cancelled), Germany, and Canada. I'd be interested in seeing examples from other countries. And if anyone can read German, perhaps you can translate the back of the German card. I was surprised at how restrictive the Canadian card was; obviously different countries had different standards.
Here is an Austro-Hungarian field post card with a Hungarian cancel. Notice that on the left, seven different languages are given for "sender". There is no printing on the back, only the sender's handwritten message.