It is restrictive: high fat, very low carb, adequate protein. However, it is comparatively easy to maintain since a high-fat diet leaves you feeling much fuller.
After the adaptation phase (~3weeks) your metabolism shifts to becoming much more efficient at burning fat. As a result if you go on a slight calorie deficit, the rest of the calories come from stored fat and you don't feel hungry.
Many people do these diets for nutritional health (myself included). There is lots of new research about it and while its nutritional benefits may be debated --its well understood to be the most effective way to loose fat sustainability.
ketotic.org is a good place to start research on it. It has research summaries on ketogenic diets by Zooko Wilcox O'Hearn (a well-known cryptographer and open-source dev of Tahoe-LAFS) and Amber Wilcox O'Hearn, a researcher who found that a stricter variant of a ketogenic diet completely cured a major health issue of hers.
reddit.com/r/keto is actually the best place to start once you've decided you interested. They have a good FAQ and the focus is mostly weight loss.