All matter is made of atoms, which are millions of times smaller than the smallest object we can see without a microscope (and thousands of times smaller than the smallest object we can see with an optical microscope).

Web site showing the scale of an atom here.

An atom consists of a nucleus (protons and neutrons) in the centre, and a 'cloud' of electrons in the space around it. Most of the atom is empty space - the radius of the atom (as shown in the scale web site linked above) is 10,000x larger than the radius of the nucleus.

Protons and neutrons have about the same mass, about 1836x the mass of an electron.
Protons and electrons have the same magnitude of charge, but protons are positive and electrons are negative. Neutrons have no charge.

Electricity is the flow of charge.
A material that allows flow of charge is called a conductor; an insulator does not allow the flow of charge.
A metal is a good conductor because it has delocalised electrons which are free to move through the metal. A solution of ions is also a good conductor as the ions are charged and can easily flow through the solution.

Flash animation of how metals conduct electricity here.

Opposite electric charges (positive and negative) cause a potential difference (voltage). Potential difference is energy per charge; stronger opposite charges will cause greater force on charges and therefore each will carry more energy. Potential difference is measured in JC^-1 (joules per coulomb).
Electric current is the rate of flow of charge. If more charge is flowing past a point each second, the current is higher. By convention, current flow is from positive to negative (the opposite of electron flow). Current is measured in Cs^-1 (coulombs per second).