Not QUITE that simple - there are people who can spin slower than 90rpm.
zeWoo, I think what you're looking for is this...
Mashing is when you stomp on the pedals like you're walking up stairs! So you are only using the "down" part of the stroke with any power. When you get clipless pedals or toe clips you can pull up too but still mash because you are still only using the vertical parts of the stroke.
Spinning is about using your muscles to make sure that you are using the full circular revolution to push the bike forward. So this means you do push down at that part of the stroke, but you also pull up on the up part of the stroke, push forward over the top of the pedal stroke and drag back as your foot goes through the bottom of the stroke (or instead of stroke I should really say circle).
This is why all these helpful people suggest one-legged drills which show you how to smooth out your muscles in each leg to deliver your power nice and smoothly through all these parts of the circle your foot is moving in. And when you get it nice and smooth you can spin at higher cadences for longer and get more for less!
Funnily enough, I first learnt about spinning by reading Trixie Belden books when I was a teenager!

The characters in the book went on a biking tour and one of the boys took the girls aside and told them about "ankling" which is the process of flexing your foot as you pedal (upward as you go through the top of the stroke and downward as you go through the bottom of the stroke) to enxtend the range of the circle through which you have pressure on the pedals. Wonder what Nancy Drew taught me???
This is a good foundation for spinning as it starts you using more of the circle. I recently did a ten day tour on a tandem with my ten year-old daughter and her mashing drove me nuts for the first day or two, as it really reverberates through the bike. I had plenty of time to chat with her about it (!?) so I talked to her about mashing and spinning. As it turned out the most effective way for her to stop mashing was for her to think about "doing circles" with her feet. In no time she was spinning like a beaut and anytime I got that stomping kind of feeling coming through, I would just say "circles" and she'd smooth it out.
I think a lot of the mashing vs spinning switch mechanism is in the mind!

And the rest is muscle memory. Don't wait to try it - try it now and get good habits from the start and then you won't have to undo the mashing issues!