ComfyUI is a locally-run UI (i.e. you need to have a beefy local GPU) that's generally more-complicated to learn than the older, more-common Automatic1111. However, it works kind of like some professional image-processing software does, lets you create a directed acyclic graph, lets one build up complicated workflows using those. It's also got some nice features, like the ability to queue up multiple requests (though it's not the only UI to do this, it's kind of a glaring limitation in Automatic1111). ComfyUI also caches what it has generated while traversing the DAG, and only regenerates what data is necessary. It's also capable of doing Flux, which Automatic1111 is not, and while Flux is not today a full replacement for everything I have done with Stable Diffusion, it can do a *lot* and has a far better ability to understand natural-language descriptions of scenes and less propensity to generate mangled images. As a result, I generally use ComfyUI rather than Automatic1111 now; it's a more "serious" tool for building up complex stuff.