There are two different account types associated with Minecraft: Java Edition; Minecraft accounts and Mojang accounts. Minecraft accounts were the first way to log into Minecraft on a computer, and Mojang accounts were later introduced as a single account to use across all Mojang games. The following describes the differences between the account types.
 

Mojang Account

• Currently, all Minecraft: Java Edition purchases are attached to a Mojang account.
• You have registered a Mojang account and log into account.mojang.com with your email address.
• After you make your purchase, you can select a Minecraft username and log into the game.

Mojang Account (migrated from Minecraft account) 

• You have migrated your paid Minecraft account to a Mojang account.
• You use your email address to log into Minecraft and Mojang services.
• You no longer log into Minecraft and minecraft.net with a username.

Minecraft account - paid (formerly known as premium) 

• You have purchased a copy of Minecraft from Minecraft.net.
• You use your Minecraft username to log into Minecraft services, and can access the full game.
• You can reset the password and migrate the account to a Mojang account by visiting account.mojang.com/migrate.

Minecraft account - unpaid 

• You have previously registered a Minecraft username but did not purchase the game.
• You use your Minecraft username to log into Minecraft services, but are unable to use services other than demo mode.
• These accounts were deleted in February 2015, and the usernames were made available to all, except those accounts that had purchased a gift code.