We use different kinds of insulin to more closely mimic the blood sugar control of a person without diabetes. Our bodies produce and release some insulin all the time, and more at meals. When your doctor gives you one long acting insulin, that mimics the “all the time” insulin, and the rapid-acting or regular insulin are the “more at meals” insulin. We call the “more at meals” (or at other times when your blood sugar is high) a “bolus”. Just means “extra”, really.
Insulin is a hormone - a protein - which moves from your fat into your blood through tiny holes in the capillary walls, and then throughout the body, where it leaves through tiny holes in the capillary walls.
In an emergency, a person **can **be given insulin straight into their vein through an IV, but you run the risk of lowering their blood sugar too much or too fast, causing hypoglycemia. Hypoglycemia can cause Really Bad Things, including coma and seizures, so IV insulin is pretty rare, even in hospitals. By using insulin subcutaneously - into the fat pad under your skin - it releases at a much slower and more gradual rate, and is far easier to control without making you hypoglycemic.
Regular insulin takes about 30 minutes to an hour to get to all your cells, which is why you often inject it before you make your meal. Rapid acting insulin takes only 15 minutes to get to all your cells, which is why you don’t inject it until you have food on the table in front of you. If you take it and don’t eat, you may become dangerously hypoglycemic.
Long acting insulin takes, on average, 1-3 hours to begin to get to all your cells, so you don’t have to worry about timing it to your food. Rather, you take it at the same time every day.
There are three ways in which regular insulin is changed into long acting insulin. All of these are designed to slow down the absorption step.
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Lente crystalline zinc insulin (Ultralente, NPH or Lente insulines). They add zinc to the insulin. This zinc affects the chemical structure so that the insulin breaks down a bit (into dimers and hexamers - smaller chemical chains) at the site of injection, and that makes it absorb more slowly into your system.
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Insulin glargine. This is an insulin molecule that, with recombinant DNA technology, has had 2 arganine molecules attached in one spot and one asparagine molecule taken out and replaced with a glycine. So it’s not exactly insulin anymore, and we call in an “insulin analogue”. It acts like insulin where we want it to, but it’s got a more neutral pH. More neutral pH means that it’s absorbed out of your injection site more slowly.
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Insulin detemir (Levemir). This is an insulin molecule that has a fatty acyl chain bonded to it. Again, this changes the molecule, so we can’t call it “insulin” anymore; it’s another insulin analogue. This fatty acly chain makes it attach itself to the albumin in your body. Albumin is a protein in your blood and on your cells. The insulin detemir attaches to it and then peels away slowly, again slowing the time it takes to get fully absorbed.
Other chemical changes are made to insulin to make rapid acting insulin analogues, like Lispro and Aspart. But since you didn’t ask about them, I’m going to skip the chemistry. 
Suffice it to say that those chemical changes make the insulin analogues absorb more quickly than regular insulin. That makes it good for meal coverage, but not effective for long term glucose control. If you used just rapid acting insulin, your blood glucose level would yo-yo all day, and that’s not what we want. That way lies peripheral vascular disease and amputations.
The current standard of practice is to use both a long term and a bolus of regular or rapid acting insulin in the amount they’re needed in order to keep the patient’s blood glucose as even and close to 100 as possible.
