What's Januvia all about? (In plain english)
Simply put, Januvia works by decreasing the amount of sugar produced by the liver. In most type II diabetics the liver produces too much, for reasons only partly understood. Januvia also makes the pancreas produce more insulin in response to high blood sugar. This mechanism is also defective in type II diabetes, again for reasons poorly understood. Januvia does these things by a new mechanism of action, and is the first drug that affects the first problem listed above.
So we have these alpha & beta cells who aren't doing what they're supposed to do - they're producing too much glucose (or not preventing the liver from doing so), so the body's natural insulin isn't enough. So, when that happens, it would be good if the "incretin" system kicked in to regulate these naughty cells - but DPP-4 normally stops the system doing that (to a degree). Januvia stops the DPP-4 that stops the incretin stopping the dysfunctional cells, meaning Januvia indirectly stops your these cells from producing too much glucose.
Put another way, it's an upstream inhibitor of glucagon. Glucagon signals the body that it has low blood sugar. It tells the liver to produce sugars in response, because the body thinks you're in a fasting state. In a normal person glucagon is inhibited when you eat food, because insulin is released. Insulin tells the body - 'It's Dinner Time!!' - and you're liver production of sugar stops as blood sugar is used up. Apparently this system gets screwed up in people with diabetes, as the balancing act between insulin and glucagon doesn't work properly. Therefore this medication will help the body realize, that when blood sugar is high, to stop liver production of sugars (and possibly tell the pancreas to release insulin), which should aid diabetics in controlling blood sugar levels.