From Diablo Wiki
→Ladder Characters: patch number
A Ladder Character is a special character type that is allowed to compete on the ladder. These characters are separate from Normal Characters with their own games and economy.
All characters created before 1.
10 are considered normal characters. Characters created before 1. 10 do not have the option of being ladder characters. You must create a new character after patch 1. 10 is released, and mark it as a ladder character to create a ladder character.
Characters created before patch 1.
10 will not be able to join games with these new ladder characters, although they will be able to play with other normal characters that predate the 1. 10 patch or characters created after 1. 10 that are not created as ladder characters. This means that for the duration of the ladder season - which is yet to be determined - these ladder characters will exist in a new separate economy from the other Battle.net Diablo II characters. After the ladder season is over, all ladder characters become normal characters. To be on the ladder for the next season, players must create a new character and mark it as ladder, after the new season has begun.
Only Realm Characters can be Ladder Characters.
* Ladder Core: Characters can only join games with other Ladder Core characters. They cannot join games with Hardcore characters, or normal characters of any type.
* Ladder Hardcore: Characters can only join games with other Ladder Hardcore characters. They cannot join games with core characters, or normal characters of any type. To create a Ladder Hardcore Character you must finish the game on Normal Difficulty.
=What's the advantage to creating a Ladder Character?=