Gehinnom, aka hell, is the level that God created on the second day of Creation. It is under the control of the other-side and where the wicked souls go through their ‘punishment’ / Purification.
The normal purification process called ‘whitening’ in Gehinnom takes 12 months, but some wicked may get stuck in there for a longer period.
The Garden of Eden is higher level, made for the righteous, and Gehinnom is for the wicked in the opposite direction below.
The fire in Gehinnom burns day and night like the fire of selfish desires.
When the souls get ‘whitened,’ the impurity of their past actions remains in Gehinnom as boiling feces. The wicked who didn’t repent are doomed there in boiling feces and never leave.
More about Gehinom in Teruma paragraphs #440+, Pikudei #842, #853, #866, #882, Vayikra #274, more….

Synonyms:
Gehenna, gehinnom, גיהנם, גהינום , Gehennam