aes() function is located in package ggplot2. It performs aesthetic mapping of dataset to the plotting. That is, how variables in the data are mapped to visual properties (aesthetics) of geoms. Aesthetic mappings can be set in ggplot() and in individual layers.

aes(x, y, ...)

x, y, ...: List of name-value pairs in the form aesthetic = variable describing which variables in the layer data should be mapped to which aesthetics used by the paired geom/stat. The names for x and y can be omitted; all other aesthetics must be named.

> install.packages("ggplot2",repos="http://cran.r-project.org",dep=TRUE)
> library(ggplot2)
> gg <- ggplot(CO2, aes(Plant,uptake))  #CO2 is a builtin dataset
> gg + geom_boxplot(aes(color=Type))