A ladder character is any character you make after patch 1.10, a non ladder character ofcouse would be any character make before patch 1.10.

A ladder character have all the new bouses and new items that Blizzard add on, but A non-ladder character will only recive some of those bouses.

You can't join a Ladder character game, if you playing a non-ladder character, ofcouse the oppsite way. There will be a time when you can join ladder character game, it's when the END OF LADDER SEASON, but the end of ladder season could be atleast 6 month to never come.

Hope I answer your question.