Electronic – What does the reference designator DB mean when printed on a PCB

78K2120 is the Newark product code for that diode.

http://www.newark.com/vishay-general-semiconductor/1n4004-e3/diode/dp/78K2120?Ntt=78K2120

Google FTW.

That diode's temperature range is -50~+150 degrees celsius. Data sheet: http://www.vishay.com/doc?88503; Farnell Australia (au.farnell.com) part number 1421919.

The technical term for the markings is "reference designators" (aka "refdes") and there are a few standards can define them. Take a look at this wikipedia page for a quick overview. http://en.wikipedia.org/wiki/Electronic_symbol

http://blogs.mentor.com/tom-hausherr/blog/tag/reference-designator/

For schematic components, most EDA tools start off with one or few alphabets and then a sequential number. For example, R1 for the first resistor, C1 for the first capacitor, IC1 for the first IC and so on. You can download a free EDA tool such as Eagle to play around. Also, see the wikipedia page for a few more examples.

For PCB footprints, different vendors do make naming convention suggestions. See Altium's suggestions here, for example.

Edit: I do NOT know anyone personally that refers to this as a strict standard or a standard at all. It's mostly what you are used to and familiar with.