What is the Minecraft scoreboard command to display words?

You cannot change the type of value shown on the sidebar in vanilla; it will always be a name followed by an integer score.

However, you can use vanilla's team prefix/suffix feature to display text before or after the names of those within the team (which includes fake players/non-player entities).

In-game

Create scoreboard teams to represent ranks.

/scoreboard teams add VIP
/scoreboard teams add Diamond

Insert players into the appropriate teams.

/scoreboard teams join VIP @p

Create a dummy objective to show on the sidebar and initialize players with a score of 0. The score itself won't matter unless you want to sort players in a specific manner.

/scoreboard objectives add Display dummy
/scoreboard objectives setdisplay sidebar Display
/scoreboard players set @a Display 0

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However, if you wanted to order it based on rank, you can give players with a higher rank a higher score:

/scoreboard players set @a[team=VIP] Display 2
/scoreboard players set @a[team=Diamond] Display 1

Editing team prefix/suffix

Exit the world. Navigate to the <world name>/data/scoreboard.dat file and open it with an NBT editor such as NBTExplorer.

Open the data compound, the Teams list, and then locate the teams you want to modify. Edit the prefix/suffix to include text for display, which can include section-symbol-formatted codes.

Final result

When you enter the world again, your name will include the additional text: