How would you describe in laymen terms to somebody what the differences is between the Rijksmuseum vs Louvre vs Uffizi Gallery? How would you classify the type of content inside the museum?
Uffizi Gallery is an art gallery, mostly focused on italian art and with a special focus on Renaissance. I can't find the number of objects in the exposition, but it's probably the smallest of the three.
The Rijksmuseum is both an art, a craft and an history museum, meaning that its display of about 8000 objects include both works of arts (paintings, sculptures) and objects of historical importance. And it's a slightly more geared toward Dutch art and history, obviously.
Finally, the Louvre is a huge art, craft and history museum (I mean, really huge: it's the largest museum in the world) with nearly 400.000 objects on display from all the world and all the eras.
Having to explain them any further, or having to suggest them in some way, I'd say that if you plan to see only one museum in your life among those three, you either are really 100% only interested only in italian stuff and thus you visit the Uffizi, or you go a couple days to the Louvre.
I've only been to the Louvre.
The Louvre is BIG; plan at least a couple of days to see everything or plan your visit to only see specific halls.
The Louvre has a lot of diverse exhibitions, Antiquities ( Egyptian , Roman, Greece), a "new-ish" Muslim art section, decorative arts...
There are sculpture halls and painting halls, from many eras and tons of other stuff all over the place.
As far as I can see, the Rijksmuseum and the Uffizi are smaller and more focused on paintings.
I suggest looking at their individual web sites to find out more about them.