How to save a data.frame in R?

I made a data.frame in R that is not very big, but it takes quite some time to build. I would to save it as a file, which I can than again open in R?

There are several ways. One way is to use save() to save the exact object. e.g. for data frame foo:

save(foo,file="data.Rda")

Then load it with:

load("data.Rda")

You could also use write.table() or something like that to save the table in plain text, or dput() to obtain R code to reproduce the table.

If you are only saving a single object (your data frame), you could also use saveRDS.
To save:

saveRDS(foo, file="data.Rda")

Then read it with:

bar <- readRDS(file="data.Rda")

The difference between saveRDS and save is that in the former only one object can be saved and the name of the object is not forced to be the same after you load it.