Horizontal boring mills come in two types - Floor type and table type.  Both will have a spindle that moves towards the part on a plane horizontal to the floor.  Most modern table type boring mills will be smaller tables with less weight capacity, but may have a rotation axis that allows them to turn and index like a lazy susan.  These are what the workpiece is fixed to.  Floor types are generally for larger pieces.  Some of them can run on rails that take up a whole room.

Another type of boring mill is the vertical boring mill, which is generally the same thing as a vertical turret lathe.  The difference is that the whole base of the machine will normally function like a chuck, rotating the part fixed to it to be turned (or machined in more modern CNC machines) and cut.