The Forces
Everything that goes on in the universe can be accounted for by one of four forces. If you've ever taken a physics class, you probably learned about a lot of different forces, such as gravity, magnetism, friction, tension, electric forces, and so on. But some of these forces are caused, deep down, by the same things, and when you finally reduce it down to ones that are truly different, it comes down to four: gravity, electromagnetism (see how we've combined some here), and the subnuclear forces called the strong and weak force.
Gravity
You're pretty familiar with this one. Gravity is the force that holds planets and stars together, keeps the planets in orbit around the sun, makes things that go up come down again, and gives us the tides.
Electromagnetism
Electricity and magnetism were originally thought to be completely separate phenomena. Then Hans Christian Oersted discovered that an electric current can change the direction of a nearby compass needle, and the first link between the two forces was discovered. Over the rest of the nineteenth century, more and stronger connections were found between the two, until today they're not considered separate forces at all by physicists.
Electromagnetism (mostly electricity) is responsible for a lot more of our world than you probably realize. It all stems from the fact that the electrons in an atom are held near the nucleus by electric forces. The forces that binds atoms to each other (or holds them apart) are also electric forces. And so you and everything around you is held together by electric forces. The force that keeps you from sinking into the floor when you walk is a collection of electric forces between the atoms in the floor and the atoms in your shoes or your feet. Any kind of force that you may notice on a macroscopic (i.e. everyday-sized) scale that isn't gravity is electromagnetism.
The Strong Force
The strong force is, as its name implies, a very strong force. However, it plays only a small (but very important) role in what goes on around us. The strong force binds protons and neutrons together to form the nuclei of atoms. Now the protons all carry a positive electric charge, which means that they all repel each other (electrically). This means that any force which tends to pack them tightly together must be considerably stronger than the electric force that tries to pry them apart. Hence the name, the strong force.
The Weak Force
The weak force has an even smaller role in our lives, although still a very important one. For us, its most important job is done in the sun and other stars. In the sun, hydrogen is converted to helium, a process that releases energy, which reaches earth in the form of light and heat. The transformation takes several stages, some of which involve a conversion of protons to neutrons. Of the four forces, the weak force is the only one that can bring this about. The weak force is very weak, however, and so the processes in the sun which involve the weak force take place far less frequently than they would if the weak force were as strong as, say, electromagnetic forces. This is what gives stars their great longevity.
Et cetera
These forces do a lot more things than the ones I've mentioned here. I just picked out a few of the more everyday functions of these forces to use as examples. I mean, do you really care about b-quark decay or Compton scattering?
The Particles
Dave's Microcosmos