In the context of Harry Potter I would imagine H/D = Harry/Draco?

The alien race in Homestuck (who call themselves "trolls", yes this does lead to confusion) has four kinds of relationships they consider romantic, also known as "romantic quadrants". Very short version: red romance/heart quadrant is equivalent to human romance, pale romance/diamond quadrant is a non-sexual BFFs-keeping-each-other-sane sort of thing, black romance/spades quadrant is attraction based on competition, ashen romance/clover quadrant is one person mediating between two others so they don't end up in murder.

The romantic ideal of the trolls is to have "all your quadrants filled" each with a different person. As you can imagine, this has given the shippers a wealth of new material to apply both to Homestuck characters and other canons.