Since perhaps the time SMW hacking took off, hacks have generally been classified as either Vanilla, Chocolate, or ChocoNilla. All this really tells potential players is whether or not there's custom content in the hack. I'm not saying the flavor system should be completely scrapped, but it can easily be supplemented with a second system that further defines the hacks in question.
Traditional
A traditional hack is a hack that aims to feel like an official Mario game. Story is very minimal and is usually able to be summarized in the text box on the Intro Screen of the game. Naturally, most vanilla hacks are traditional, but it's not uncommon for a chocolate hack to also fit into this style if most of the custom content consists of graphics, sprites, music, and blocks, with most applied patches doing little to alter gameplay beyond what's normal for Mario platformers.
Puzzle
As the name implies, puzzle hacks require the player to solve puzzles to proceed, rather than simply run through levels jumping on heads, eating mushrooms, and shooting fireballs. While a puzzle hack can possibly be created with vanilla resources, it's much more common for said hacks to use custom ASM-based resources to add more variety to the puzzles. Examples of puzzle hacks include Mario's Keytastrophe and Karoshi Mario.
Adventure
Adventure hacks have more elaborate stories that are often told through cutscenes. These hacks are rarely vanilla, as the VWF Cutscene Tool is typically used to create the story-advancing cutscenes. These hacks will also use NPC sprites and may also have RPG elements such as a coins-as-currency system. The best example of an adventure hack would be SMW2+3: The Essence Star
Gimmick
These hacks focus on elaborate ASM-based gimmicks and almost serve as tech demos to show the extent to which Super Mario World can be hacked by someone with enough skill in 65c816 Assembly code. Most of these hacks come from Japan, but there could be one out there somewhere that doesn't. A good example of a gimmick hack would be Brutal Mario.
Transformation
These hacks replace Mario with a different character, either from another game or, more frequently, a character made by the author him/herself. These are almost NEVER vanilla hacks, as almost everything in the game is changed, from graphics to physics. This category may be more of a subcategory than anything, as these hacks can have the narrative simplicity of a traditional hack, the epic feel of an adventure hack, the brain-tease of a puzzle hack, or the wow factor of a gimmick hack. Examples of Transformation hacks include Just Blob and Saph and the Great Rebellion.
Kaizo
Unlike the other categories, this one has existed for as long as SMW hacking itself. If you don't know what a Kaizo hack is, it's basically a hack that takes its difficulty to the very limits of human capability. TV Tropes refers to such hacks as Platform Hell games.
Quintesson Judge: Silence, or you will be held in contempt of this court!
Hot Rod: I have nothing but contempt for this court!
- Transformers the Movie (1986)
Traditional
A traditional hack is a hack that aims to feel like an official Mario game. Story is very minimal and is usually able to be summarized in the text box on the Intro Screen of the game. Naturally, most vanilla hacks are traditional, but it's not uncommon for a chocolate hack to also fit into this style if most of the custom content consists of graphics, sprites, music, and blocks, with most applied patches doing little to alter gameplay beyond what's normal for Mario platformers.
Puzzle
As the name implies, puzzle hacks require the player to solve puzzles to proceed, rather than simply run through levels jumping on heads, eating mushrooms, and shooting fireballs. While a puzzle hack can possibly be created with vanilla resources, it's much more common for said hacks to use custom ASM-based resources to add more variety to the puzzles. Examples of puzzle hacks include Mario's Keytastrophe and Karoshi Mario.
Adventure
Adventure hacks have more elaborate stories that are often told through cutscenes. These hacks are rarely vanilla, as the VWF Cutscene Tool is typically used to create the story-advancing cutscenes. These hacks will also use NPC sprites and may also have RPG elements such as a coins-as-currency system. The best example of an adventure hack would be SMW2+3: The Essence Star
Gimmick
These hacks focus on elaborate ASM-based gimmicks and almost serve as tech demos to show the extent to which Super Mario World can be hacked by someone with enough skill in 65c816 Assembly code. Most of these hacks come from Japan, but there could be one out there somewhere that doesn't. A good example of a gimmick hack would be Brutal Mario.
Transformation
These hacks replace Mario with a different character, either from another game or, more frequently, a character made by the author him/herself. These are almost NEVER vanilla hacks, as almost everything in the game is changed, from graphics to physics. This category may be more of a subcategory than anything, as these hacks can have the narrative simplicity of a traditional hack, the epic feel of an adventure hack, the brain-tease of a puzzle hack, or the wow factor of a gimmick hack. Examples of Transformation hacks include Just Blob and Saph and the Great Rebellion.
Kaizo
Unlike the other categories, this one has existed for as long as SMW hacking itself. If you don't know what a Kaizo hack is, it's basically a hack that takes its difficulty to the very limits of human capability. TV Tropes refers to such hacks as Platform Hell games.
Quintesson Judge: Silence, or you will be held in contempt of this court!
Hot Rod: I have nothing but contempt for this court!
- Transformers the Movie (1986)