What is the difference between an associate agent and a just plain agent? Is it anything like being a sub-agent or a junior agent?
Are there just a lot of rungs in the ladder, each with its own title?
Not really. Associate/Junior/Sub-agent generally all mean the same thing - "We've come up with a fancy title for the person who is not the head agent." Don't worry about it. If you've found someone who sells your work to a publisher, you're golden.