Caffenol is a homemade black-and-white film developer. It’s popular with some enthusiasts because it can be made from relatively common, less toxic ingredients and is often considered more environmentally friendly than some conventional developers.

In general, caffenol formulas rely on:

• a source of caffeic acid, usually instant coffee or sometimes tea
• a pH booster, commonly sodium carbonate (washing soda)
• often vitamin C (ascorbic acid), depending on the recipe

There isn’t one single formula—many recipes exist. One example shared in the community answer uses instant coffee crystals, washing soda, and ascorbic acid for the developer, with diluted white vinegar as a stop bath.

You don’t usually buy “caffenol” as a standard commercial product; instead, you mix it yourself from a recipe. If you want to try it, search for established caffenol recipes and follow one carefully, since proportions and processing times vary.

It’s used much like other film developers: mix the solution, develop the film, stop development, then continue with the rest of the normal film-processing workflow.