What is the use of a *.pb file in TensorFlow and how does it work?

pb stands for protobuf. In TensorFlow, the protbuf file contains the graph definition as well as the weights of the model. Thus, a pb file is all you need to be able to run a given trained model.

Given a pb file, you can load it as follow.

def load_pb(path_to_pb):
    with tf.gfile.GFile(path_to_pb, "rb") as f:
        graph_def = tf.GraphDef()
        graph_def.ParseFromString(f.read())
    with tf.Graph().as_default() as graph:
        tf.import_graph_def(graph_def, name="")
        return graph

Once you have loaded the graph, you can basically do anything. For instance, you can retrieve tensors of interest with

input = graph.get_tensor_by_name('input:0')
output = graph.get_tensor_by_name('output:0')

and use regular TensorFlow routine like:

sess.run(output, feed_dict={input: some_data})