What is a ladder character?

This is a confusing topic, for sure.

The "Ladder" in Diablo 2 is essentially a scoreboard that keeps track of the top characters in each realm. Occasionally, there is a Ladder reset, where all Ladder characters are integrated into non-Ladder and Ladder characters must be created anew.

Since Ladder characters cannot interact at all with non-Ladder characters, this means there are two separate economies going at once. Ladder characters do not have access to older items that may have been created before patches changed item drop rates or stats, or old items made by item duplication exploits. Because of this, good items tend to be much more expensive in Ladder.

Additionally, there are some items that are only available in Ladder, and cannot be dropped or created (usually via a Rune Word) in non-Ladder. These items continue to exist, however, when the Ladder characters are integrated with the non-Ladder during a reset, and these items can be quite valuable in the non-Ladder economy.

Overall, Ladder in Diablo 2 is a way for Blizzard to help control the economy. If the economy is getting out of hand, a new Ladder season will just start the economy over because everything is removed and has to be found from scratch again.