The difference is the order of cart and horse. You can end up in a similar place, but you get there by either the DM taking the lead or the PCs.
I'm like Ghost; a "Situations" guy. I'll start the PCs off in a town or keep and present the world to them. Rumors, recent events, etc. From there it's up to them to decide on a direction and goal. I have no idea at all where the campaign is going, and I have no visualization of how it's going to end or who the main BBEG is going to be. That's all developed during the course of "adventuring", which can often start as simple as "Let's see what's over that hill" or "Caves, eh? I wonder what's in there?" but often leads to very specific goal oriented campaigns like "The Empire of Ish needs to go. Let's overthrow it." or "The Tharzidun cultists are clearly making a major push to take over the Kingdom of Bren. They must be stopped."
But in all cases the PCs make those choices, not me. Naturally this doesn't work if the PCs are passive and wait for me to tell them what "the Adventure" is going to be for the evening. They need to have motives and goals of their own.
There's a lot of "winging it" involved, as The Ghost mentioned, due to the uncertainty of direction, but I'm fairly prepared for most eventualities. I have a monster deck (3x5 index cards of all the common monsters in the area), tons of generic maps (from 100-mile scale to 5' scale), a gross of random generators and a good imagination.
I do find that the DMing style evolves away from "Situation" towards more "planned Adventure" over time, but that's only because I have a better sense of what the PCs want to do and where they want to go, so I can plan ahead better. I'm also able to flesh out a dungeon between sessions better if the PCs have decided to go into it and stop gaming for the night mid-delve. But at no point have to told, bribed or asked them (via NPC or sotto voice) to go that way, and they're free to change tactics or goals at any time.
-------------
"Adventures" are like most of the adventures you could have bought from Wizards recently. PCs show up and are presented with a mission of some sort by an NPC. It's not a 'bad' way to run D&D, but it's certainly different than how I run it.