By default, R treats strings as factors (categorical variables). In some situations, you may want to leave them as character strings. Use stringsAsFactors=FALSE to achieve this:
> auto <- read.csv("auto-mpg.csv",stringsAsFactors=FALSE)
However, to selectively treat variables as characters, you can load the file with the defaults (that is, read all strings as factors) and then use as.character() to convert the requisite factor variables to characters.