Heterochromatin is a transcriptionally silent packaged form of chromatin, and euchromatin is a transcriptionally active form of chromatin. Having these two forms of chromatin allows for the entire genome of an eukaryotic organism to fit into the nucleus.
As heterochromatin is very tightly packaged, it is not possible for transcription machinery to access genes, and so is perceived to be transcriptionally silent. It is possible to loop out genes with gene loops, allowing the transcription of genes in the heterochromatin, moving the d-loop to a transcription factory to transcribe it into pre-mRNA. Constitutive heterochromatin is inherited from parental cells, being maintained through cell division, and therefore being harder to access and express. Facultative heterochromatin is less tightly packaged than constitutive heterochromatin, and can be decondensed to allow transcription. Although heterochromatin can be identified through nuclear imaging, it is now possible to use advanced sequencing technology to identify DNA bound to epigenetic markers - these can be used to determine transcriptionally active and inactive regions of the chromatin.
Euchromatin is less tightly packaged, allowing histones and other proteins bound to the DNA to be pushed along by the transcription machinery. This enables euchromatic regions to be more actively transcribed. Housekeeping genes would be within euchromatic regions.