The junco's pale pink beak is a key identifier, as is the strong contrast between the dark gray or brown top and the white belly. Males and females are similar, though the females' color is more muted. There are regional variations in coloration -- so much so that until the 1980s they were treated as several distinct species. The Cornell Lab of Ornithology notes:
A field guide is the best place to look for complete illustration of ranges and plumages, but in general there are two widespread forms of the Dark-eyed Junco: “slate-colored” junco of the eastern United States and most of Canada, which is smooth gray above; and “Oregon” junco, found across much of the western U.S., with a dark hood, warm brown back and rufous flanks.Juncos eat seeds and insects and usually feed on the ground, as is typical for sparrows, and so they don't often come to our feeders, though they are often to be seen foraging underneath them. They are regular visitors to our yard, though typically not in large numbers.
Yesterday morning we got a few inches of new snow from the edge of the big storm that shut down Iowa and other midwestern states. The snow covered the typical seed litter under our feeders, and while I was at home at lunchtime, I saw a junco trying to get a peanut from our peanut feeder. That's not a sight I've ever seen before. So I swept away the snow from a section of our front walk and put out some seed on the ground for the juncos.
They are cute little birds, and we're always happy to see them.