Q: What's a talent agent?
A: A talent agent finds auditions for their actor-clients by speaking directly with casting directors and/or accessing online breakdown services that publish audition information. He/she fields and filters audition requirements to the actor (i.e. date, time, what to prepare) and then negotiates the actor's contract should he/she book the job. An actor can either freelance with an agent (which means he/she does not need to work with them exclusively) or sign with an agency (agency = bunch of agents; this means the actor works with this agency exclusively; contract length varies, but usually begins as one year). Agents are paid when their clients book jobs, receiving a percentage of the actor's weekly salary.
Q: What's a secret agent?
A: A secret agent is someone who wears well-tailored clothes, carries gadgets, speaks in spy-talk (i.e. "transmitting intel in T minus oh-eight hours"), and fights evil organizations.
Q: What's a casting director?
A: A casting director works for a casting company and seeks out actors for various auditions. They are hired by production teams looking to cast a new or ongoing project. In most cases, the casting director does the initial screening of the actor candidates, then recommends, or calls back, actors whom he/she feels the production team should take a look at. Their expertise is manifold: not only must they have a thorough handle on the material for which they are casting, but they must also be highly perceptive in determining the best types of people to "fit" the role. Additionally, they must dip into their ken of experience and knowledge of working actors when determining who to call in for the audition. In short: talent agents (see above) submit their clients to casting directors; casting directors decide whom to let in the gate.
Q: What's a cruise director?
A: A cruise director does stuff like stage sing-alongs at midnight buffets. Somebody like this guy. And God have mercy on their pitiful, seafaring souls.