A Casting Director (CD) is a person that is hired by the producers of a show, commercial, or print ad etc. to find talent for a project. Casting Directors are the people who agents will send photos and resumes to, and the actor (you) will audition for. It’s the Casting Director’s job to find the best possible talent out of thousands of actors and models submitted for each role. A CD wants you to do well so they look good as well. Usually, a good CD will walk you through the audition process so you do your best job.
If there's a Casting Director that you'd like to meet with it's a good idea to mail them your headshot and resume. It’s also a good idea to send postcards to the CD’s with your recent projects. I usually do this every couple months. It’s good to get your face out there on a regular basis, eventually the casting director will bring you in to audition for a part that’s right for you.
It’s possible to submit your photo and resume directly to a Casting Director if you know of a role being cast, however they usually only consider submissions by an agent or actors they already know. The best way to get to know a casting director is by auditioning for another role with that CD or you can set up an "interview" which is a tuff process with all the talent out there and everyone wanting to meet with them.