Open Neural Network Exchange (ONNX) is an open standard format for representing machine learning models. While there are proprietary formats such as pickle (for Python) and MOJO (for H20 AI), there was a need to drive interoperability.
ONNX defines a common set of operators - the building blocks of machine learning and deep learning models - and a common file format to enable AI developers to use models with a variety of frameworks, tools, runtimes, and compilers. ONNX also provides a definition of an extensible computation graph model and each computation dataflow graph is structured as a list of nodes that form an acyclic graph. Nodes have one or more inputs and one or more outputs. Each node is a call to an operator. The entire source code of the standard is available here - https://github.com/onnx/
Thus ONNX enables an open ecosystem for interoperable AI models. The ONNX Model Zoo is a collection of pre-trained, state-of-the-art models in the ONNX format that can be easily reused in a plethora of AI frameworks. All popular AI frameworks such as TensorFlow, CoreML, Caffe2, PyTorch, Keras, etc. support ONNX.
There are also opensource tools that enable us to convert existing models into ONNX format - https://github.com/onnx/onnxmltools