There is a must-read article by Matt Strassler:
In my opinion you should read the comments section first and then read the article. This is a disturbing article. The commentators call it a “shocker.” I was also taken aback even though I was a physicist specializing in accelerators and colliders. After reading Matt Strassler’s tutorial on the true nature of protons and neutrons, I realized that my nuclear physics knowledge was very out of date.
I am developing my own models of elementary particles and I always emphasize that most physics theories are just models. This is why I wrote “Model of a Model” to remind every one that physics models/theories are removed from reality by at least 2 levels. So, my models of elementary particles are just models. I know that but most scientists are not careful about this distinction when they talk about their models.
The second point you should remember is that protons and neutrons are not elementary particles. When I say I am developing models for elementary particles I am referring to truly elementary non-composite particles such as electrons and neutrinos. I do not claim any expertise on the composite particles such as protons and neutrons.
Protons and neutrons are not elementary but quarks are. Quarks are very mysterious. Perhaps more mysterious than neutrinos!
In his tutorial article Matt Strassler explains that physicist understanding of quarks evolved since they were first proposed.
“The history of science is full of partial answers to questions that have to be revised over time, or struggles between conflicting versions of a theoretical idea. 1960′s quarks were a great idea that led to great progress, but it took a while to figure out what they were, and weren’t. It is 1970′s quarks — the ones that are very light [specifically, the up and down quarks are light], and interact with gluons through the equations known as Quantum Chromodynamics — that go into the Standard Model of particle physics. And there are an enormous number of them (along with antiquarks and gluons) in the proton.” – Matt Strassler
The shocker in Strassler’s article is the explanation that inside a proton there are actually more than 3 quarks.
I highly recommend that you read the comments section of his article first.