I'll begin discussion of the elements of this diagram by talking about the actors. Actors An actor is a role that a user plays with respect to the system. There will probably be many traders in the given organization, but as far as the system is concerned, they all play the same role. A user may also play more than one role. When dealing with actors, it is important to think about roles rather than people or job titles.
Actors carry out use cases.
In practice, I find that actors are most useful when trying to come up with the use cases. Faced with a big system, it can often be difficult to come up with a list of use cases. It is easier in those situations to arrive at the list of actors first, and then try to work out the use cases for each actor. Note that actors don't need to be human, even though actors are represented as stick figures within a use case diagram. An actor can also be an external system that needs some information from the current system.