Data frames are special lists that can also store tabular values. However, there is a constraint on the length of elements in the lists: they all have to be of a similar length. You can consider every element in the list as columns, and their lengths can be considered as rows.

Just like lists, a data frame can have objects belonging to different classes in a column; this was not allowed in matrices.

Let's quickly create a data frame using the data.frame() function:

a <- c(1, 3, 5)
b <- c("red", "yellow", "blue")
c <- c(TRUE, FALSE, TRUE)
df <- data.frame(a, b, c) 
df
##   a      b     c
##   1    red  TRUE
##   3 yellow FALSE
##   5   blue  TRUE

You can see the headers of a table as a, b, and c; they are the column names. Every line of the table represents a row, starting with the name of each row.