Simply put, a major is student's area of expertise (English, Biology, Economics, etc.) and is required. A concentration is more like a minor with a liberal arts spirit and is not required.

Like a minor, a concentration requires a smaller course load. Unlike a minor, it is designed to bring multiple disciplines together.

So, for example, a chemistry major with a biology minor will take mainly biology courses to fulfill her minor requirements. Meanwhile, a chemistry major with an environmental studies concentration might take a biology course on "Population Ecology," a geology course on "Geochemistry of Natural Waters," a religion course on "Buddhism and Ecology," and a history course on "Farm and Forest: African Environmental History" to fulfill his requirements.