Many classes incorporate the Spellcasting feature. One, Warlock, instead has the feature Pact Magic. What are the mechanical differences between the two, or is this just a distinction without a difference? The multiclass rules treat Spellcasting and Pact Magic slots interchangably, and warlocks are small-s spellcasters for the purposes of attunement....
They're quite different...
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.
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.
1\$\begingroup\$ I think the last bullet for each outlines what I'd consider the major difference between Spellcasting and Pact Magic; a Warlock, as far as I understand it, cannot cast a spell as a lower level spell once they get spell slots for a higher level (i.e. a 3rd level Warlock can no longer cast Hex or whatever as a 1st level spell; it's essentially a 2nd level spell now and that's it, until they hit 5th level, at which point it becomes a 3rd level spell, etc...) \$\endgroup\$– NathanSDec 14, 2017 at 14:43
2\$\begingroup\$ Some of the traits listed for Pact Magic are blurred whether they belong to Pact Magic or the Warlock Class. Since at this moment (dec / 2017) they are one and the same, it is not important, but might become in the future if they decide to reuse the feature in another class. \$\endgroup\$ Dec 14, 2017 at 17:37
The difference in name reflects (and calls attention to) the slightly different way in which pact magic works. Likely so players don't just skip reading that section if they've read spellcasting for another class. The main differences are that for Pact Magic there is only one size of spell slot (which increases in size at certain levels) and the spell slots recharge after a long OR short rest.
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. For example, when you are 5th level, you have two 3rd-level spell slots. To cast the 1st-level spell thunderwave, you must spend one of those slots, and you cast it as a 3rd-level spell (PHB 107)
In comparison, with spellcasting the caster has access to spell slots of multiple levels and they only recharge after a long rest
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. For example, if you know the 1st-level spell burning hands and have a 1st-level and a 2nd-level spell slot available, you can cast burning hands using either slot (PHB 101)
The example above is from the Sorcerer section but applies in the same way to all classes with spellcasting.
As far as I know, the only other significant difference is that warlocks tend to get less spell slots (as they recharge on a short or long rest) but have more powerful cantrips (ie eldritch blast) and are capable martial fighters to make up for this.
It's also worth noting that if your DM takes a similar stance to mine on short rests (they're largely unnecessary or unrealistic when in a dungeon) and does things to interfere with your short rests, this can mean a warlock only has two or three spells for a whole day