The structural protein that DNA wraps around - something very important if you are a 10 micrometer cheek cell and you need to package over a meter of DNA. This is roughly a million fold difference in length (obviously, the cell has volume as well) so compression is important. Only eukaryotes, with their millions and billions of base pairs have histones although there are homologs in some non-nuclear species.

Unsurprisingly, the sequence of the histone is very well conserved as it probably cannot tolerate much change to structure. DNA looks much the same no matter what it codes for (although methylation makes a difference) and presumably wraps around the histone octamer in much the same way. This supercoiling of the helix is affected by the sequence of bases - but the binding is generally sequence independant. Since genes have to be 'unpacked' to be read, the chromatin (histone-DNA) structure has to be dynamic to allow transcription machinery access.

How the histones are packaged up into higher order complexes is largely unknown.