This is yet another term for what is more commonly known as Generation Y or the Baby Boom Echo — the children of the baby boomers. As is usually the case with these demographic terms, there is debate about the actual birth years involved. The starting points I've seen range from 1978 to 1982, and the ending points range from 1982(!) to 2003(!).

This phrase is often shortened to just Millennials, a term that first came about in 1991:

Finally, today's (Civic) Millennial tots, born after 1981, are already the stars of cuddly baby movies and the objects of wartime sympathy. America's next great cadre of doers, they will grow up basking in adult praise for their intelligence, obedience and optimism. They will happily participate in a national service a Boom-run Congress is likely to establish for them. But rising Millennials will be encouraged to build more than reflect. By the time today's infants reach age 30, spiritual energy will be the province of the old, not the young. —William Strauss, "The 'Constellation' of 1991'," The Washington Post, February 24, 1991