To save you the effort:
There are four fundamental forces in physics:
Gravity, Electromagnetic, and the Strong and Weak Nuclear Forces.
They have very different strengths. For illustration, imagine dropping a ball onto a concrete floor. Gravity accelerates the ball towards the ground, however, just before it hits, the electrons which make up the outer parts of the atoms of the ball and floor strongly repel each other, bringing about a considerably sharper acceleration. This stops the ball dead almost instantaneously. The Electromagnetic force is therefore considerably stronger than the force of Gravity.
Now, in the nucleus, the centre of the atom, there's a bundle of protons and neutrons, the former having positive charges and the latter none. If no other forces existed, the EM force would cause them to fly apart at huge velocity. However, the Strong nuclear force is much, much, much stronger than even the EM force. This "glues" the protons and neutrons [similar types of particles] together, and forms the nucleus
You may ask why all the protons in the universe don't all rush together suddenly. The reason for this is that the forces are effective over different ranges. The solar system is held together over millions of miles by gravity. The electrons are held in the atom over distances of around an angstrom [0.000000001m] by the EM force, and the protons are held in the nucleus by the Strong Nuclear force over a distance of around a few femtometre's [0.000000000000001m].
Hope this helps,
Nick