ButtonBar[{lbl_1 :> act_1, lbl_2 :> act_2, ...}] represents a bar of buttons with labels lbl_i that perform actions act_i when pressed.

ButtonBox[boxes] is a low-level box construct that represents a button in a notebook expression.

ButtonBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for cells that specifies settings for buttons within the cell.

ButtonData is an option for the low-level function ButtonBox that specifies the second argument to give to the ButtonFunction for the button when the button is active and is ...

ButtonFunction is an option for the low-level function ButtonBox that specifies the function to execute when the button is active and is clicked.

ButtonMinHeight is an option for the low-level function ButtonBox that specifies the minimum total height in units of font size that should be allowed for the button.

Button[label, action] represents a button that is labeled with label, and evaluates action whenever it is clicked.

ButtonNotebook[] gives the notebook, if any, that contains the button which initiated the current evaluation.

ButtonSource is an option for the low-level function ButtonBox that specifies the first argument to give to the ButtonFunction for the button when the button is active and is ...

ByteCount[expr] gives the number of bytes used internally by Mathematica to store expr.