head(): See the first five rows of a dataframe
citation(): Get the citation for R if nothing is included in the parentheses. If a package name is included in parentheses in quotations, you will get the citation for that package.
merge(x = , y = , by = ): Merge dataframes x and y by their common column (specified with by =). Using the default setting, only rows common to both x and y will be retained in the merged dataframe.