Yes, your impression is pretty much correct, although there has been more research done on mindfulness than guided imagery. Still the outcomes on stress, performance, depression, pain, anxiety and quality of life appear to be comparable.
Mindfulness meditation is a way of constantly bringing attention to our internal experience, and focusing on things like the sensation of breathing; the way the inside of the body feels; and/or noticing thoughts, impressions, sensations, emotions, perceptions and feelings, as fleeting as they may be, as they register on your mind.
The idea is that of course the mind can’t stay focused on any of these things for more than a second or two – it jumps around of its own accord (- and as a result is frequently referred to as “monkey mind” by meditators). But the practice has you continually noticing what the mind is doing and gently bringing its focus back to one or more of those designated internal experiences.
What happens is that over time, this has a calming effect. It centers you. The more aware of your internal experience you are, the better able you are to be strong and steady within yourself, and less at the whim of other people and external events. Your ability to focus and concentrate improves. And as your body awareness gets more acute, your early warning system for problems or symptoms gets strengthened. You’re better able to notice when something isn’t quite right and get it looked at, taken care of. In other words, steady, disciplined practice makes most people braver, smarter, stronger and more effective.
Guided imagery requires less discipline and practice. In essence, you hit the Play button and focus on the voice, the music and the evocative language and images, and it carries you. That’s why it’s been called the lazy man’s (or woman’s) meditation.
The voice and images may in fact take you inside to your own inner experience, the way mindfulness does; but it can also provide a more goal-directed, structured kind of imaginal “story board” narrative of what you’re hoping for – a desired end-state, such as more confidence, less pain, a successful medical procedure, a reduction of anxiety, a smooth and stellar athletic performance, and so on.
Because it’s easier to do and takes less focus and discipline, it can be the gateway to other forms of meditating or it can stand alone as the way to go. There’s no one silver bullet for everyone, and some prefer one over the other; some do both.
I hope that explains it. Try both and see what suits you. You may be surprised at what you discover.