Hell modes are One Step From Edens version of ascension, stacking difficulty mods that you unlock by beating the previous one.

1. Enemies start with random artifacts.
2. More obstacles and hazards.
3. Lower defense, no aim crosshair.
4. Spared bosses healing halved. (200 instead of 400)
5. Max HP is reduced. (75% of default)
6. Enemies start with 2 random artifacts.
7. Luck starts at 20. (up from 3)
8. Enemies regenerate health.
9. Bosses start at tier 2.
10. Max mana reduced by 1.
11. Hostages are evil.
12. Shops cost more.
13. All bosses are max tier.
14. Start with 1 max HP. (For shopkeeper, your max HP will be 100 due to starting gold, but you will have 1HP to start.)

My tip for progressing through hell modes is to find a couple of builds you like, and stick with them.
My go-to builds were spellpower, poison+shields and flow. In my experience gunner and reva were the easiest characters to progress with, but goodluck doing hellmode 14 with anyone other than shopkeeper.