The histone code isn't a sequence of letters like you see in DNA and RNA but rather refers to the pattern of post-translational modification of histones. This is then, by definition, an epigenetic control of gene expression.
Histones are basic DNA binding proteins around which DNA is wound to form chromatin. For transcription to occur, transcription factors and RNA polymerase must bind DNA, which generally cannot happen if DNA is wrapped tightly around histones. DNA must therefore be able to come off from and translocate relative to histones in order to expose bare regions. Based on the level of compaction, chromatin can be generally classified as two types:
- euchromatin: loosely bound, transcriptionally active
- heterochromatin: tightly bound, transcriptionally repressed
The state of chromatin compaction is determined by specific post-translational modifications to the residues in the histones' amino-terminal tail. This is the histone code. One commonly talked about and perhaps easiest to understand modifications is acetylation of lysine residues: this is generally considered to activate transcription since it neutralizes the positive charge of the amino acid so the negatively charged DNA isn't attracted as tightly. Phosphorylation of serine or threonine can have a similar effect by introducing negative charges. Other modifications include mono-, di- and trimethylation of lysine and arginine, ubiquitinylation and others that I haven't memorised. Each histone monomer can be modified independently and each has several residues that can be modified; this leads to staggeringly large number of ways in which just a single nucleosome can be modified. Importantly, these modifications can recruit proteins including transcription factors, histone modifiers and chromatin remodellers (which can condense or loosen the chromatin). All of these serve to tightly regulate gene expression. This is an active field of research and what all discovered combinations of modifications do is largely unknown, but certain modifications are often characteristic of active and repressed transcription.
There is a standard convention for conveying histone modifications. For example, if lysine 23 of histone H3 was acetylated, you would write H3K23Ac. For trimethylation of arginine 3 on histone H4, you would write H4R3Me3.