I think it's pretty easy to explain relatively unambiguously though.
When you're looking at an object, let's say an orange, you are experiencing a unique, particular sensation which usually only happens when you're looking at an orange. Being able to visualize things is to be able to create the same sensation wilfully even when you're not looking at an orange (albeit of perhaps a lesser intensity).
It can also be used to conjure a short movie like sequence or animation as well. I most frequently use it to plan routes in my mind whether its in the car or how I'm going to move through a particular space it helps orient you and improves reaction speed and can be tied to muscle memory. It takes some focus and is rarely used simultaneously with other activities. It's more like a planning or focusing activity for brief bursts before you take action.
Its actually quite like those slow motion 360 sequences in the matrix. I think that's probably why that film technique gained such traction, as its a slowed down version of what our brains do when we use our visual memory to navigate our movement.
What's the difference between imagining something and seeing a picture? That's kind of rhetorical, and kind of not. The article that this page links to from 2005 says:
"Asked to imagine a familiar item or recall a recent event, most people can call to mind an image that is less vivid than the original but has a visual ‘feel’. This expresses our capacity to experience visual mental imagery."
Which is still super vague as to what people do when they "call to mind" something, or what a visual feel is...