Most of the common connectors you use to connect networking equipment use DB type connectors.
I always understood that the ‘D’ was because the shape of the connector is like a letter D.
The ‘B’ I think is used to show the number of pins, as are other letters between A and F, but commonly everyone uses ‘B’ and none of the other letters which would be more correct for some sizes of connector.
The number at the end is also the total number of pins possible (some may be missing or not connected).
So in common use, a DB-25 is the good old 25-pin RS-232 connector you find on old modems, a DB-15 is the X.21 connector (and that games port on your PC), and a DB-9 is the 9 pin serial connector used on PC serial ports such as laptops, or the PC end of the Cisco console cable.