In what sense is the "histone code" a code?
As already mentioned by Stefan, the "histone" code is not really a universal code; it is restricted to eukaryotic systems and even then, not unambiguous. It is, at present related to two kinds of histone modifications and their correlation with the transcriptional activity at that locus:


Methylation
Acetylation


Other modifications such as phosphorylation and ubiquitylation also exist but they are not generally considered in this "code".

Acetylation masks the histone's positive charge and thereby loosens the chromatin (DNA wrapped around the histones), which in turn facilitates transcription. Therefore acetylation is almost always activating in the sense of gene expression.

Methylation, on the other hand has different effects. Methylation on Histone-3 at 4th lysine (H3K4) and H3K36 causes activation whereas H3K9 and H3K27 are repressive. There is also some difference between different extent of methylation i.e monomethylation, dimethylation and trimethylation. The effect of methylation is executed by proteins that have domains such as chromodomain and PHD fingers, that identify these modified histones. Sometimes the histone can carry both kinds of methylation marks H3K4me3 and H3K27me3 and these are called bivalents. 

Other histones of the octameric nucleosome can also be modified. However as with other kinds of modifications that I mentioned previously they do not seem to be very common and are therefore not generally considered in this "code".