They aren't sweet. They don't have a starchy character. Both add body and help in head retention. They don't have any flavor, or a barely perceptible one.
Weyermann describes CaraFoam as a 2-row Caramel malt roasted to about 2 °L that improves a beer's foam quality, head retention, and body. While it's recommended CaraFoam make up 5-10% of the grist to achieve the intended effect, it can apparently be used at up to 40% without causing detriment to the beer.