Weapons
A melee weapon is something found on the simple melee weapons table or the martial melee weapons table. You hold it and hit your foe with it. You can also throw it at them, though it might not be very effective when thrown.
A ranged weapon is something found on the simple ranged weapons table or the martial ranged weapons table. You either throw it at a foe (if it has the thrown property) or use it to launch some sort of missile at the foe (if it has the ammunition property). You can hold it and hit a foe with it but it will not be very effective.
From the Combat chapter of the rules:
Hitting a target within your reach with an item that you are holding is a melee weapon attack. The ability modifier used is Strength.
Throwing an item at a foe or launching some sort of missile at them is a ranged weapon attack. The ability modifier used is Dexterity.
Thrown property
If a melee weapon has the thrown property then throwing it at a foe is making a ranged weapon attack but using Strength, not Dexterity. The weapon is still a melee weapon (not a ranged weapon), which is important for some features.
If a melee weapon doesn't have the thrown property, you can still throw it but you won't be as effective. It is considered an improvised weapon and will do 1d4 + DEX Mod damage. Your ranged weapon attack is with an item that is not on any of the weapon tables (improvised weapons are not actual weapons), so it uses Dexterity.
If a ranged weapon has the thrown property then throwing it at a foe is making a ranged weapon attack using Dexterity.
Finesse property
If a weapon (melee or ranged) has the finesse property then you can choose to use Strength or Dexterity for melee weapon attacks and ranged weapon attacks made with it, though you must use the same modifier for both the attack roll and the damage roll.
Other
There are also a few things that aren't weapons but are still used to make a melee weapon attack (using Strength) or a ranged weapon attack (using Dexterity): unarmed attacks, claw/bite/etc attacks and improvised weapons.
Summary
• "Melee weapon" and "ranged weapon" is based on which table the weapon appear in. Melee weapons use Strength; ranged weapons use Dexterity.
• "Melee weapon attack" and "ranged weapon attack" are based on what you are doing.
| What are you doing? | Melee Weapon | Melee Weapon (thrown) |
Ranged Weapon (thrown) |
Ranged Weapon (ammunition) |
|---|---|---|---|---|
| Hitting someone while holding it | Melee weapon attack Strength |
Melee weapon attack Strength |
Improvised melee weapon attack Strength |
Improvised melee weapon attack Strength |
| Throwing it | Improvised ranged weapon attack Dexterity |
Ranged weapon attack Strength |
Ranged weapon attack Dexterity |
Improvised ranged weapon attack Dexterity |
| Launching ammunition from it | N/A | N/A | N/A | Ranged weapon attack Dexterity |
• If the weapon has the finesse tag then you can choose to use either Strength or Dexterity, regardless of the type of attack.
The game is pretty consistent about the difference between "attack with a melee weapon" and "melee weapon attack". For example, Barbarian Rage damage bonus requires a "melee weapon attack using Strength" and therefore doesn't apply to thrown melee weapons or thrown ranged weapons.
