Here's the simple version of insulin resistance (others can probably give you more scientific info):
When we eat, most of what we eat eventually turns into glucose. Some foods get converted quickly (like simple sugars) and some take much longer to get converted (protein). At any rate, our cells use glucose for fuel...whether it's the cells that make up muscle, brain cells...they all need glucose to function. But all cells have a "door" through which the glucose enters. Insulin acts like a "key" to open the "door". If you have insulin resistance, it's like having ten locks, instead of just one, on the door, so you need much more insulin(more keys) to open the door. If the door isn't opened, then instead of the glucose being utilized by the cells, it floats around in the blood stream and doesn't get absorbed into the cells. Thus, your blood glucose is high, and this can do damage in many ways.
Insulin resistance is a factor in Type II diabetes, where the body does produce some insulin, but it's not enough to open the cells and process the blood glucose. People who are insulin resistant can be treated with any number of medications, some of which cause the cells to open more easily, and some of which increase the body's production of insulin. Also, being overweight (or more accurately, having too high a body mass index) increases the insulin resistance of the cells, which is why diet and exercise alone work for some Type II diabetics.
In Type I diabetes, there simply is no insulin being produced at all, so there is no choice...with none being produced by your body, you must take insulin to survive.
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