From WoodworkersZone WoodWiki
A horizontal router table differs from a "normal" router table in the orientation of the router. Instead of having the axis of the router being oriented vertically, the router is "laid on its side" horizontally, thus the name.
Horizontal router tables are much less popular, especially commercially, than vertical ones. For work where the stock would be balanced on edge using a vertical table, the horizontal router table can excel.
Example horizontal router table:
The router side:
In use, making a tenon: