"Nethack classes" also commonly refers to the role of your character (e.g. Valkyrie or Healer) as distinct from their race (human, elf,...) These are the character classes you can play in Nethack.

Advantages: Starting with speed, stealth, a pick-axe and a tinning kit. The ability to identify gems
Disadvantages: Bad starting armour and weapons, poor physical statistics
Difficulty: High

Advantages: High strength and constitution, good starting weapon, poison resistance
Disadvantages: Fairly poor starting armour
Difficulty: Low

Advantages: High strength and constitution
Disadvantages: No useful equipment or intrinsics
Difficulty: Moderate

Advantages: Poison resistance, lots of hit points, healing spells
Disadvantages: Poor starting armour and weapons, low strength
Difficulty: High

Advantages: Good starting weapons and armour (better in Slash'EM), ability to jump, can obtain Excalibur
Disadvantages: No useful intrinsics except speed, pony as starting pet
Difficulty: Moderate

Advantages: Reasonably good starting armour, lots of intrinsics
Disadvantages: Can't wear body armour, martial arts not as good as an artifact weapon
Difficulty: High (low in Slash'EM)

Advantages: Automatic detection of blessed or cursed items
Disadvantages: No intrinsics, don't start with much food
Difficulty: Moderate

Advantages: Starting with lots of food and a bow and arrows
Disadvantages: Fairly poor starting hand-to-hand weapon
Difficulty: Moderate

Advantages: Starting with a sack and a lock pick
Disadvantages: Poor starting armour and weapon
Difficulty: High

Advantages: Speed, good starting weapons, good physical statistics
Disadvantages: Don't start with much food, fairly tough quest nemesis
Difficulty: Low

Advantages: Starting with lots of food and gold, very useful quest artifact
Disadvantages: Useless starting armour and weapons, no intrinsics, poor statistics
Difficulty: High

Advantages: Good starting weapon and armour, cold resistance and stealth, can obtain Excalibur if lawful, first gift is Mjollnir
Disadvantages: Not much good at spellcasting, quest artifact is heavy
Difficulty: Low

Advantages: Starting with lots of magical items and magic resistance, superb at spellcasting
Disadvantages: No intrinsics, poor starting armour, poor physical statistics
Difficulty: Moderate