Violin plots are kernel density estimates mirrored in the vertical plane. They can be used to visualize several distributions side-by-side, with the mirroring helping to highlight any differences.
ggplot(diamonds, aes(cut, price)) +
geom_violin()
Violin plots are named for their resemblance to the musical instrument, this is particularly visible when they are coupled with an overlaid boxplot. This visualisation then describes the underlying distributions both in terms of Tukey's 5 number summary (as boxplots) and full continuous density estimates (violins).
ggplot(diamonds, aes(cut, price)) +
geom_violin() +
geom_boxplot(width = .1, fill = "black", outlier.shape = NA) +
stat_summary(fun.y = "median", geom = "point", col = "white")