HAT, or HP - AC - TWENTY SIDED, is a roleplaying game that shares three basic concepts with D&D but diverges from there. The three concepts kept from D&D are obvious from the title: HP, AC, and the D20 roll. However, each is expanded in scope. One of the goals of HAT is to create a game that embraces a more narrative and situation driven feel, while maintaining support for classic D&D modules. This is the first version on these rules, known affectionately by the author as the Bowler edition.
HP is now more than just Hit Points: Hunger Points. Its the drive that keeps the character going, and wanting more. Injury saps them in all forms, mental, magical, and physical. When a character runs out, they simply no longer have the ambition to keep on being an adventurer.
AC is also expanded to mean more than just Armor. It is still Armor Class, and it is still a measure of defense. However now it means more than just physical defense, but mental and magical and so on too. However it no longer functions to resist an attack, but to lessen the effect of such.
The D20 roll remains the same, but only in the most basic way. You still roll a D20 and add a modifier to get a result yes, but instead of trying to reach a target number you earn levels of outcome - one for every five rolled points. Its outcome that matters in the end here, and that is compared to other ratings to see what happens as a result of the roll. Also, sometimes you will roll more than one dice and keep the highest, when your adventurer has advantage.
Here are quick links into the online wiki for HAT.