Chromatin is compossed of DNA and proteins. 
-Beads on a String:  The DNA is negatively charged and loops two times around the postively charged proteins (called Histones: H2A, H2B, H3, and H4).  There are 2 of each of these histones (to make an octamer).  Another protein (H1) connects the octamer beads to each other.

Heterochromatin: condensed and inactive (no transcription)
Euchromatin: less condensed and active (transcription happening on it)