One of the main differences between whole wheat and whole grain is the process that is used to prepare the grain flour. With whole-wheat flour, the grain has gone through a refining process that has removed some of the nutritional value from the end product. By contrast, whole-grain flour does not go through this refining process, and thus maintains the natural level of nutrients.
"100% whole wheat" refers to minimally processed wheat berries along w/ their outer husks, for added fiber. The husk is not removed nor is the wheat berry polished to change it's color. "Whole grain" is similar. The grain - oats, rye, barely, quinoa, etc. is also minimally processed & often coarsely chopped & added to foods, like breads & cereals. For more roughage. But whole grains are also milled down into refined flours, eg. Cheerios. Not ideal but still better than super refined, bleached, white wheat flour. Supposedly 100% whole wheat is a little better than products labeled "whole grain" - guarantees more concentrated fiber & nutrients. Ideally you want "100%" whole wheat" or "100% whole grain". But check the ingredient list - these two foods should be the first ingredients or bulk of the ingredients. Some products say, "Made with..." Not the same - a teaspoon of ground up grain thrown into the recipe doesn't count. It's a marketing scam.