It's quite possible you've already played a ladder campaign with a friend without even realising it. Any time you sat down with you mate after a game, thought about what would happen next and planned the next scenario around that idea, you've been playing a ladder campaign. The only difference is that a proper ladder campaign the scenarios are planned before the games are played, rather than after them.

It's important to note that all the games don't have to be the same size in points or even the same game system. It's fine for the games to start with 500pts a side, escalating upto 1500pts across the three games, or for one of the games to be run with Necromunda rules or even Aeronautic Imperialis rules Can you imagine how much fun it would be to play a couple of small scale sabotage scenarios in preparation for a large AI bombing raid? Don't be scared to give these things a go, campaigns are supposed to be liberating remember ;-)
The example makes it perfectly clear what a ladder campaign is, but it's missing the key element that makes campaigns interesting, the idea of on going effect. It makes no reference to how succeeding or failing at a mission effects the campaign overall. Ladder campaigns are quite restrictive in this aspect, as the next scenario is set by the narrative and not dictated by the results of the previous game or the tactical decisions of the players.
Ladder campaigns are a great way for friends to add a bit of fun to their games and can really help with coming up with interesting scenarios to play rather than the standard three. If you're looking for a more dynamic narrative, then tree campaigns often just that.
I hope that this gives you an insight into Ladder campaigns and what they can offer regular gaming buddies. If you've got any ideas, additions or experiences you'd like to share on this, then add them in the comments below.