[R] (no subject)

[nicolas.deig@epfl.ch]

hi,

I am trying to grow a classification tree on some data, but I have a little
problem. In order to do so I have to use a function like "tree" in R and on the
internet help(tree) I get the following: 

"The left-hand-side (response) should be either a numerical vector when a
regression tree will be fitted or a factor, when a classification tree is produced"

I would like to know what is a "factor" in R, is it numerical value with no
formula or just a word??

Thanks in advance
Nicolas