Saravanan Subramanian has an introduction to data frames in R:
The R data frame is a high level data structure which is equivalent to a table in database systems. It is highly useful to work with machine learning algorithms, and it’s very flexible and easy to use.
The standard definition of data frames are a “tightly coupled collections of variables which share many of the properties of matrices and of lists, used as the fundamental data structure by most of R‘s modeling software.”
Data frames are a powerful abstraction and make R a lot easier for database professionals than application developers who are used to thinking iteratively and with one object at a time.