Euchromatin is when DNA is not as tightly packed. It is the site for active transcription and replication where the DNA is accessible to RNA polymerases and the replication machinery. It is present in both prokaryotes and eukaryotes. In eukaryotes, DNA is wrapped around histone proteins and it looks like beads on a string. Euchromatin looks like loose beads with DNA. It can be visulized by G banding as a light band.
Heterochomatin is when DNA is tightly packed. It is usually to protect DNA. IT is only present in Eukaryotes and stain darker in GBanding.
The first figure shows the relative positions of heterochromatin and euchromatin in a typical chromosome. The second figure on the left shows how each is packaged. The third figure on the right shows an exaple karyotype ( G banding ) experiment with giesa stain. The lighter regions are euchroatin and the darer regions are more tight represents heterochromatin.