In a nutshell
Germans arriving on the American frontier in the nineteenth century brought with them pietism (religion of the heart), a mix of Lutheran and Reformed faith, missionary zeal, and a yearning for peace in reaction to the religious wars in Germany. Eventually coalescing into the Evangelical Synod of North America, it merged with the Reformed Church in the United States in 1934.
Center of gravity
Midwest.
What you might expect if you visit one
Strong sense of ethnic German heritage, perhaps including an annual service in German. Traditional church buildings that recall German architecture and decoration, with dark carved wood and stained glass. A balance of Lutheran and Reformed thinking with an emphasis on individual conscience and liberty. Casseroles.
Closest living non-UCC relatives
Lutherans.