sweet tooth; person who prefers sweets over alcohol
Often contrasted with 辛党 (からとう), a person who prefers alcohol or spicy/salty foods. The word can refer to a general preference for sweets or specifically to someone who chooses sweets instead of alcohol.
See also: 辛党
I have a sweet tooth and love cake.
He doesn't drink alcohol and orders sweets instead—a total sweet-lover.
甘党 is someone who prefers sweets over alcohol, the direct opposite of 辛党 in its alcohol-drinker sense.
Compound of 甘 (sweet) and 党 (party, faction), literally 'sweet faction'. The exact origin is uncertain, but it likely arose as a playful contrast to 辛党.