Having the tag Expansion Character means that the character has been created with the advantages of Diablo II: Lord of Destruction. With the character creation you have the choice to create an Expansion Character or a normal character. All Druids and all Assassins are Expansion Characters.
If you create a normal character, Lord of Destruction will behave as if the Expansion is not installed. A normal character can be converted into an Expansion character at any time, but an Expansion character cannot be converted back to normal.