The Strong force holds the nucleus of the atom together. It is hard to break apart a proton to see inside of it because this force is so strong. But the strong force only works with quarks. So electrons don't feel it. Quarks each have a property called color charge which electrons don't have. So we have 3 different color charges: red, green, and blue that allow quarks to be attracted to other quarks. The carrier of this force is called the gluon. There are actually 8 different kinds of gluons. They can change the color of a quark. These gluons are kind of like rubber bands that hold the quarks together. As the quarks get farther apart, the strong force actually gets stronger up until a point where things break and new particles are formed.

Learn about the Electromagnetic Force | Learn about the Weak Force