foo // bar
(* bar[foo] *)

Hold[foo // bar]
(* bar[foo] *)

Postfix[foo//bar]

foo//bar

ntn[u_] :=
 FlipView[{Style[u, 20],
   Row[{Style[ToExpression[u], 20]}, Background -> LightRed]}]

bar /: MakeBoxes[bar[foo_], StandardForm] := RowBox[{MakeBoxes[foo], "//", "bar"}]

bar[x_] := x^2

table = TableForm[
    {
    {Plus, a+b},
    {Subtract, a-b},
    {bar, Defer[a //bar]}
    },
    TableHeadings->{"function", "example"}
]