The dataset is in the "data frame" format, an array of data in which each case (here an individual) corresponds to a row, and each variable corresponds to a column. The row labels for these data frames are just row numbers, the column labels are the names of the variables.
The data are taken from "Applied Linear Regression", by Weisberg. The data are originally from a study of identical twins reared in different families, published by Cyril Burt. The variables are:
"Burt" <-
structure(list(Y = c(82, 80, 88, 108, 116, 117, 132, 71, 75,
93, 95, 88, 111, 63, 77, 86, 83, 93, 97, 87, 94, 96, 112, 113,
106, 107, 98), X = c(82, 90, 91, 115, 115, 129, 131, 78, 79,
82, 97, 100, 107, 68, 73, 81, 85, 87, 87, 93, 94, 95, 97, 97,
103, 106, 111), Class = structure(c(1, 1, 1, 1, 1, 1, 1, 2, 2,
2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3),
.Label = c("U", "M", "L"), class = "factor")),
.Names = c("Y", "X", "Class"),
row.names = c("1", "2", "3", "4", "5", "6", "7", "8",
"9", "10", "11", "12", "13", "14", "15", "16",
"17", "18", "19", "20", "21", "22", "23", "24",
"25", "26", "27"), class = "data.frame")