[RPG] Just what are the differences between Spellcasting and Pact Magic

They're quite different...

Spellcasting

Using Sorcerer as an example...

The Sorcerer table shows how many spell slots you have to cast your spells of 1st level and higher. To cast one of these sorcerer spells, you must expend a slot of the spell’s level or higher. You regain all expended spell slots when you finish a long rest. (PHB p101)

• Knowing and preparing spells varies between classes
  • Bard, Sorcerer, Ranger, Eldritch Knight, and Arcane Trickster know few and don't need to prepare. Some Rangers get a few extra known spells, based on their path.
  • Cleric, Druid, and Paladin know all the spells on their class list and prepare a selection of them each day. Clerics, Paladins, and some Druids get a few extra known spells based on their path.
  • Wizards keep a book of known spells, which theoretically can include all Wizard spells in the game. They prepare a selection of them each day.
• Recovers slots on a long rest
• Combines slots with other Spellcasting classes when multiclassing.
• Accumulates many slots when advancing.
• Slots have different levels.

Pact Magic

The Warlock table shows how many spell slots you have. The table also shows what the level of those slots is; all of your spell slots are the same level. To cast one of your warlock spells of 1st level or higher, you must expend a spell slot. You regain all expended spell slots when you finish a short or long rest. (PHB p107)

• Only one class has it, Warlock, and knows a limited selection of spells and doesn't prepare them.
• Recovers slots on a short or long rest.
• Does not combine slots when multiclassing.
• Has very few slots, relative to Spellcasting.
• All slots on a given character are the same level.