Oooh, now that is a can of worms! lol.
Low GI (glycaemic index) foods are ones that release their carbs slowly during digestion. So a slice of brown bread is lower GI than a slice of white bread because it is slower to release the glucose. However, they may well have very similar amounts of carbohydrate, so if they both have around 15 g of carbs per slice, then 15 g of carbs will still get into the bloodstream, although with white bread it may take approx 1hr, and with brown bread it may take 90 mins (all these figs are approximate, and intended to just give you the idea).
Type 2 diabetics are often advised (by well meaning health care professionals, friends and relatives to 'eat low GI' but those people don't have a handy test meter to test the foods' effects on blood glucose. If they did they would discover that the glucose still hits the blood stream and still raises the blood glucose, often by a surprising amount.
Low carb foods are foods that don't have much carbohydrate in them to start with. So there is less glucose to end up in the blood stream. You can get lower carb breads which might have anywhere from 6-14g carbs/slice, compared with the 15-20g/slice you would find in brown or white or granary breads.
Does that make sense?
Most low carb foods are also low GI, but some low GI foods can have surprising amounts of carb in them.