As I said in The Diabetes Answer Book (page 83):
Hemoglobin is a protein inside all of your red blood cells. It helps carry oxygen to your tissues to keep you healthy. Every day your body makes a few million new red blood cells in your bone marrow. These get pumped round and round your body for the next two or three months. When the red blood cells get old, your body destroys them in your spleen. Since the hemoglobin is floating around amongst the glucose in your blood, a small amount of glucose gets attached to the hemoglobin. Hemoglobin with glucose attached to it is called glycosylated hemoglobin or hemoglobin A1c (HbA1c). Brand new red cells have no glucose attached to them, but the longer they stay in your bloodstream, the more glucose gets attached. The higher the glucose level in your blood, the more HbA1c you form.
We can measure what percentage of your hemoglobin has glucose attached to it. This tells us what the average glucose level was in your blood for the previous ten to twelve weeks. To give you an idea of how to use this information, a non-diabetic person's blood glucose will vary between about 20 and 160 mg/dl (3.9-8.9 mmol/L) throughout the day. Over time about 4 to 6 percent of the person's hemoglobin will get glycosylated. So the normal range for HbA1c is about 4 to 6 percent. Now, imagine the worst possible blood glucose control. If someone let his or her blood glucose average remain at 400 to 600 mg/dl (22-33 mmol/L) for months on end, the HbA1c could rise above 20 percent. I have never seen HbA1c higher than 25 percent, but it can almost get to that level if someone leaves his or her diabetes completely untreated for long enough.
Doctors pay so much attention to HbA1c that it is important for you to understand what it is and how to interpret it. I will refer back to this in other blog posts.

