The simplest example of this: as a programmer, when your boss says to code a feature, you code it. But as a PM, you have to get a team to make it.
A PM's job is to figure out what the feature really is, and how complex or configurable it should be, and what the target audience is, and how you'll measure success. It also involves making sure that you feature works well with other features, and that your team is moving in the same direction as the rest of your company.
Be prepared, BTW, for the constant assumption that you became a PM because you couldn't hack it :-)