I think this is a good time for more concrete examples. In an adventure I planned a while back, I came up with a plot.
The villian was a baron, but eager for more power. He wanted to rule the entire kingdom. He came up with a plan, he would pay the local orcs to attack his own villages. Since he'd be completely unable to repel these sort of attacks, he'd have to ask the King for further troops under his command. He could then use these troops to attack the King and take over the kingdom as he was going to convince them that he was the rightful ruler through a combination of magic and outright lies.
Being an intelligent man, he knew there was a couple of threats to his plan, adventurers for one are good at figuring out plots like this. Plus, the King might get suspicious if it didn't look like he was TRYING to get rid of the orcs on his own. So, he put up a bunch of posters asking for adventurers to help him out.
All the PCs started at 1st level and this is where I started them out, saying they were all out of work adventurers and this was the only job they'd found in a couple of weeks. So they all went to the Duke's keep in order to get the mission.
The plan was to send them to kill the orcs, but warn the orcs they were coming in advance and pay them extra to ensure the PCs deaths.
That was the first adventure. However, I already had a basic outline of the rest of the campaign. The PCs would "defeat" the band of orcs as the orcs would underestimate them. They would find a clue, a coffer of gold with the crest of the duke on it. The duke would claim that it was stolen from him, of course.
The duke would then proceed to hail them as heroes and request that they go on more and more suicidal missions. And I'd drop hints that they might be being set up until they decided to challenge him.
If they decided to decline a mission given to them by the duke, he would start hiring assassins to kill them so they couldn't be any more threat to him.
The duke also had a powerful wizard in on the plan with him who was using magic to hide the duke's alignment and help to make sure the plot remained undetected.
So, given all of this was decided before the first session began, and I pretty much knew the general outline of the campaign (even if I didn't know the exact enemies they'd be fighting after the first adventure), and the PCs were going to be going on these quests through a combination of creative manipulation of their egos, appealing to their lawful sides, and making it sound like the missions were for good, and paying them money, and attempting to kill them if they didn't go along with it. I mean, the PCs had choice, within the confines of my planned adventure. However, the adventure was designed to steer people or BEAT them back onto the path I had planned. They could figure out the plot and take things into their own hands at any time, but if they did it too soon, they couldn't prove anything and the duke would be too powerful to them, likely ending in their own deaths.
The question is, is this sort of thing considered railroading or not?