ONNX and ONNX Runtime
Getting a model off a training framework and onto an arbitrary piece of inference hardware needs a common description both sides agree on. ONNX is that description: a standardized, framework-neutral way to write down a model graph so that PyTorch, and separately TensorRT, and separately a phone's NPU compiler, can all read the same file and agree on what it means. What actually executes that file is a different question, and conflating the two is the single most common confusion this page exists to clear up.