Glucose is a type of sugar, the main type of sugar in our blood. When someone refers to a blood sugar, they are really referring to the amount of glucose in the blood.
Diabetes, properly known as Diabetes Mellitus, simply describes a higher-than-normal glucose level in the blood. There are different causes and types of diabetes, but all have the common abnormality of a high level of glucose in the blood.
Our blood carries glucose to every cell in the body. The cells use glucose as a fuel source. Glucose enters a cell and gets metabolized (burned), giving energy to the cell. It's similar to the way a car burns gasoline in order to run. For all cells, glucose is the main source of fuel. For some cells, glucose is the only useable energy source. Such cells brain cells are a good example can't do their work without glucose.
If glucose can't get into the cell properly, it starts to accumulate in the blood, having nowhere else to go. This results in higher amounts in the blood, i.e., a high blood glucose level, (doctors refer to this as hyperglycemia: hyper - meaning higher than normal; glyc - glucose; and emia - in the blood). When it reaches a certain level we classify this as Diabetes. In the fasting state (no food or drink for the last 8 hours), a normal level of glucose in the blood is considered to be less than 6.1 mmol/L; when glucose accumulates to levels of greater than 6.9 mmol/L, in the fasting state, this is considered in Canada to be high enough to confirm the diagnosis of diabetes. There are, however, other ways to make the diagnosis (see Diagnosis of Diabetes).