What is Ladder?

Just to add some corrections to the above, season 7 didn't start yesterday. It also doesn't start tonight. It starts 2024-05-24T00:00:00   This timestamp has been automatically converted to your local date/time for convenience. The format is DD/MMM/YYYY, HH:MM am/pm. This is based off your browser time. .
See "patch" notes:
As for the OP, ladder is simply a different game mode. It is largely the same as non-ladder with the

Key difference being that ladder is functionally separate, meaning you won't have access to any non-ladder gear or characters within ladder, and ladder is divided into seasons (see above for season 7 start time). At the end of each such season, all ladder characters and gear/stash from said season are converted into non-ladder before the next season starts.

As a result, you're basically looking at "the same thing" except that every so often, everyone will be put back on even footing with everyone else as everything is reset for the new season.

Other than that, the only real difference is that ladder allows you to create the handful of ladder-only runewords available. Realistically however, that isn't too big a deal as those haven't changed for a few seasons now and as each season ends, the ones that were created on ladder are converted into non-ladder items. So while you can't create the ladder-only RWs from scratch on a non-ladder character, you can trade for them and use them on non-ladder as plenty of them have been converted down.

Back in the olden days, the item difference was a lot more significant (more top-end RWs, uniques, cube recipes, ...).
Also, back in the olden days, ladder was actually more difficult as well (as it buffed mobs a tad) which may or may not be true anymore with D2:R (never bothered to confirm) though that difference wasn't really anything to write home about either and largely unnoticeable.