In nautical terms, cardinals are danger markers in shipping channels. There are four types of cardinal, each corresponding to one of the four main points on the compass; north, east, south and west. They are painted black and yellow, have different set flashes at night, and are surmounted by two black triangles which denote the cardinal type. They are identified in the following ways;
//\\
//\\ North Cardinal
|||| Two upward triangles
|||| Black stem with yellow base
| | Constantly flashing white light
./ \.
//\\
\\// East Cardinal
|||| One upward, one downward pointing triangle (an 'East'er egg)
| | Stem is black - yellow - black
| | Flashes white three times per cycle
.///\\\.
\\//
\\// South Cardinal
| | Two downward pointing triangles
| | Yellow Stem with black base
|||| Flashes white for six short and one long per cycle (seven altogether)
.///\\\.
\\//
//\\ West Cardinal
| | One downward, one upward pointing triangle (shaped like Mae West)
|||| Stem is yellow - black - yellow
|||| Flashes white nine time per cycle
./ \.
If you encounter a cardinal marker when at sea, you should pass it keeping it on the side it marks. For example, you would stay on the north side of a north cardinal. They are commonly used to mark sandbanks, spits or obstructions in a river channels and are, like most buoys, best identified at night.
See
http://www.transport.wa.gov.au/imarine/rec_boating/pdfs/buoyage.pdf
for a better picture of the above ascii!