You want to summarize your data, based on one or more grouping variables.

Use ddply() from the plyr package with the summarise() function, and specify the operations to do:

library(MASS) # For the data set
library(plyr)

ddply(cabbages, c("Cult", "Date"), summarise, Weight = mean(HeadWt), 
      VitC = mean(VitC))

 Cult Date Weight VitC
  c39  d16   3.18 50.3
  c39  d20   2.80 49.4
  c39  d21   2.74 54.8
  c52  d16   2.26 62.5
  c52  d20   3.11 58.9
  c52  d21   1.47 71.8

Let’s take a closer look at the cabbages data set. It has two factors that can be used as grouping variables: Cult, which has levels c39 and c52, and Date, which has levels d16, d20, and d21. It also has two numeric variables, HeadWt and VitC: