The Cornell Lab of Ornithology describes them as:
...active, agile little birds with an appetite for insects and large, meaty seeds. They get their common name from their habit of jamming large nuts and acorns into tree bark, then whacking them with their sharp bill to “hatch” out the seed from the inside. White-breasted Nuthatches may be small but their voices are loud, and often their insistent nasal yammering will lead you right to them. [Link to sound clips added.]
If you click on the photo above to zoom in, you'll see that the bird is busy extracting part of a peanut from the feeder. As I've mentioned before, if you see a bird head-down on a branch, tree trunk or feeder, chances are excellent that it is a nuthatch -- either the white-breasted or its smaller relative, the red-breasted nuthatch.
Both types of nuthatches can look a bit like chickadees at a quick glance but their head-down habit is diagnostic, and nuthatches can also be distinguished from chickadees by their much shorter tails, as well as different patterns of black and white on the heads. Nuthatches have a much narrower black cap than the black-capped chickadee; white-breasted nuthatches are completely white around the eye and cheek, and red-breasted nuthatches have a black stripe through the white around around the eye.