The Greek word Gehenna refers to the eternal destination of unbelievers: hell, lake of fire, the 2nd death.

Jesus used this word 11 times. It is based on the Hebrew words for "valley" and "Hinnom".

The Valley of Hinnom was located just outside Jerusalem - the city garbage dump, filled with the stench of death & decay. Fire burned there continually.
Mark 9:48 = Isa.66:24, Rev.20:14

The King James version translated all these words as hell:  Sheol, Hades, Gehenna.

This led centuries of readers to mistakenly assume that they are all the same place.
 
Sheol appears in Gen.44:29,31; Prov.9:18; Psa.86:13, 88:3, 89:48 David & Jacob expected to go there.

The souls of everyone (believers & unbelievers) in the Old Testament went to Sheol immediately at death.
Gen.37:35; Psa.9:17; Isa.38:10

But Psa.49:15 indicates that the righteous expected to be released one day.
 
The most complete picture of Hades comes from Christ in Luke 16:19-31, The Rich Man & Lazarus.

2 men died and went to Hades.  This is not a parable but a historical record of an actual event.

In parables, the characters are not named; Jesus called them "a widow" or "a certain man".

But here, Jesus used the actual names of Abraham and Lazarus - specific not generic terms.