A walk is the simplest type of traversal you can have. It is simply a sequence of Vertex or Edge with no other stipulation. Any edge or node can be repeated.
An open walk is one where we end at a different vertex than we started at. Some traversal types like the Path are also open walks.
A closed walk is one where we end on the same vertex we started at. For some traversal types like the Circuit the only vertex which is allowed to be repeated is the first and last one, allowing it to be closed.