Total Beginner — The total beginner has little experience (if any) at all with horses in general. They may have been on a "trail ride" at a rental stable once or twice but they do not know general horse handling or the basic commands to make the horse move forward, turn, trot, stop and back unassisted. They cannot saddle or bridle a horse themselves and are not comfortable handling a horse from the ground.
Advanced Beginner - This person has a little experience with horses. They may have taken a few lessons or maybe used to ride a neighbor's horse as a child. They may not be able to saddle and bridle a horse by themselves, but they can mount and walk off unassisted. They know how to ask the horse to move forward, turn and stop.
Confident Beginner - A confident beginner has the knowledge of an advanced beginner, but will also be able to handle a horse that may not be overly willing to do as asked. Sometimes an older horse that is well broke may still balk at leaving the barn or be a little reluctant to leave a secure place. This rider will have the confidence to give a little kick if needed or use a more pervasive aid when required even though they may lack experience. They may or may not be able to post or rise to the trot. They are willing to learn and have no "fear" of horses.
Novice - A novice rider has some pretty good experience with horses. They have had a few lessons, maybe owned a horse as a child (or recently), but have not competed or trained horses. They can catch, halter, groom, saddle and bride a horse by themselves. They can mount and ride off unassisted. A novice may or may not be able to rise (or post) to the trot, but they can trot without "bouncing" and can stay comfortable with a slow canter on a gentle, well broke horse. They should know how to ask and obtain a slow controlled walk, trot/jQg and canter/lope. They can change direction and circle their horse. They are learning what a diagonal is and leads are. They may have even started a little jumping and are comfortable on a well broke horse, but may not be comfortable on a greener, younger or less experienced mount.
Intermediate - The intermediate rider has taken lessons for a while, rides in a specific discipline (or has experience in several disciplines) and may compete. He or she has ridden several different types of horses and can independently manage a horse while riding and for general care. Their seat is secure, they do not apply unintentional aids to the horse when they lose balance or become unseated. They know how to rise or post to the trot and ask for and obtain a specific lead or change in lead. They are capable of riding a less experienced horse and helping in that horses training. They are able to train/compete at a more advanced level with a trainer's assistance. The intermediate rider is knowledgeable about different horse breeds and disciplines. He or she is learning basic horse conformation.
Advanced - An advanced rider has ridden most of their lives, under a trainer's care or at least several years with intense riding instruction. They have competed successfully at recognized shows in their discipline. They are able to ride most horses (well broke or green) without assistance. They can walk, trot, canter/lope, change direction and leads. The advanced rider is able to teach lessons to beginners, train green/inexperienced horses and teach a horse more advanced maneuvers. The advanced rider knows horse breeds and conformation well and is able to detect unsoundness vs, blemishes in a horse.
Professional - The professional is paid to ride horses. They have studied under masters and are able to break horses, train and handle problem horses. The professional makes his or her living from horses. They are able to teach both horse and rider and have themselves competed in high level equine sport.
I hope these descriptions help you. Most people over estimate their riding abilities and end up with a horse they are unable to handle/train. The best thing you can do is admit your capabilities, strive to learn more and find a horse that knows a bit more than you to help you learn with confidence and safety.