The table is a list of lists. When you write data[[2]], you are asking for the 2nd part. At the top level, data is a list, so the 2nd part is the 2nd row, which is also a list. If you look up Part, you will see that giving it a list, as in {1,3,4}, asks for a list of those parts, which is the rows.

There are more complicated ways to use Part. data[[2,3]] asks for the element in the 3rd column of the 2nd row. data[[All,2]] asks for the 2nd element of All the rows.

Notice that the only thing that distinguishes rows from columns is convention. By convention the first index of a table or matrix designates a row, and the second a column. Following that convention, Mathematica displays a table that way.

Look up the function Part. The double brackets below are a way of using Part.

data = Table[{i, j}, {i, 1, 7}, {j, 1, 4}]

(*
{{{1,1},{1,2},{1,3},{1,4}},{{2,1},{2,2},{2,3},{2,4}},{{3,1},{3,2},{3,\
3},{3,4}},{{4,1},{4,2},{4,3},{4,4}},{{5,1},{5,2},{5,3},{5,4}},{{6,1},{\
6,2},{6,3},{6,4}},{{7,1},{7,2},{7,3},{7,4}}}
*)

(* take rows 1, 3, 4 *)
data[[{1, 3, 4}]]

(*
{{{1,1},{1,2},{1,3},{1,4}},{{3,1},{3,2},{3,3},{3,4}},{{4,1},{4,2},{4,\
3},{4,4}}}
*)