Type 2 diabetes is all about the inability to use insulin efficiently, and about relative shortages of insulin.
Insulin is a hormone that allows cells all over your body to make use of glucose (sugar) in your blood, for energy. Insulin is made by our pancreas (we only have one). It starts out with a reserve capacity, it can make much more than we need.
If we lose the capacity to use insulin efficiently, we are said to have insulin resistance. We can have resistance and not know it, as long as our bodies can still make enough insulin. As soon as our need for insulin increases beyond the capacity of our pancreas to produce it, the amount of glucose in our blood goes up. At that point, symptoms may be present, and the rise in glucose can be measured, this is the first indication most people have that there is a problem. Symptoms vary, but the most common is thirst.
Exercise helps lower our blood glucose, and reduce insulin resistance. Eating fewer carbohydrates results in a need for less insulin. Pills and injectable medicines help reduce our resistance, slow our digestion, regulate how much glucose our liver produces, or induce our pancreas to produce more insulin.
These medicines are taken in combination, as needed.
Type 2 is said to be progressive, that means as time goes on your pancreas makes less and less insulin. This means that as time goes on more has to be done to keep your glucose at acceptable levels. This may involve more exercise, eating fewer carbohydrates, higher doses of medication and more of them.
Usually at some point diet, exercise, pills and other injectables no longer provide enough help. It's not that they don't work any more, it's just that your pancreas can no longer produce enough insulin even with their help. At that point, it is time to take insulin.
Type 2's often take slow acting insulin, once or twice a day, this is called basal insulin. This seems to be enough about half the time. It lowers your glucose, but still requires your pancreas to help out at mealtimes. If you need more help than that, sometimes you are put on an insulin that is a mixture of slow and fast acting insulin. This also involves 2 injections a day, at mealtimes. While this can provide better results, for it to work right you need to eat the same amount of carbohydrates every day, and at the same time, basically you are matching your food to your insulin.
A more flexible and accurate way is to take fast acting insulin (called Bolus) with each meal, and Basal insulin once or twice a day. This is called MDI (Multiple Daily Injections). The amount of bolus is varied to match the amount of carbohydrates you eat.
Another way of delivering insulin is via insulin pump. There, fast acting insulin in small amounts is delivered at all times, and a larger amount of fast acting is delivered when you eat. A pump does not think for itself, you have to tell it what insulin you want delivered, but you do this ahead of time for basal. The pump stores instructions, and once it is adjusted properly, you tell it when you are about to eat (and how many grams of carbohydrate you will be eating), and the pump takes care of the rest.
In some situations a pump is much better than injections. Examples are when you need very small amounts of insulin, and when the amount of insulin you need varies by time of day, independent of what you eat.
Can a Type 2 become a type 1? Type 1 is an autoimmune attack on your pancreas, which can happen to a T2 but is very rare. Needing to take insulin does not make you a type 1. If antibodies are found, that is proof of an autoimmune attack. If this happened you would be called a "double diabetic" but officially Type 1 with resistance.
Can a Type 1 become a type 2? No. But a type 1 can acquire insulin resistance, and have most of the problems a type 2 has, in addition to type 1 problems. They would also be called a "double diabetic", but officially Type 1 with resistance.
The higher your average blood glucose, the more likely you are to have problems with your eyes, kidneys, and nerves. These problems usually take years to develop.