Ollama is a user-friendly UI for LLM inference. It is powered by llama.cpp (or a fork of it) which is more power-user oriented and requires command-line wrangling. GGML is the math library behind llama.cpp and GGUF is the associated file format used for storing LLM weights.

i've found llama.cpp (as i understand it, ollama now uses their own version of this) to work much better in practice, faster and much more flexible.