Minecraft data files are saved in the same location per computer user account. Therefore, if multiple Minecraft usernames log in via the same computer user account, they will be able to access each others' saved worlds, have access to the same items and inventory, and will have the same achievements.
If you have more than one Minecraft account, only one computer, and you'd like to keep the save files separate, you can create another user account on your computer and have each Minecraft account log into the computer on a different user account.
If you have multiple people who want to play on the same computer, and world save and player data separation is not an issue, you can play Minecraft from a single account. If you would like the world and player files separate, or would like to be known by a different username while on multiplayer servers, you may want to consider purchasing a second Minecraft account.