Atom is the smallest piece of matter that can take part in a chemical reaction. Atom consists of a nucleus in the centre and electrons spinning "around" him. The electrons are not moving in specific tracks. Though it is more possible to be in some areas and less to be in others.
The nucleus consists of protons and neutrons and the biggest part of the mass of the atom is concentrated here. Very strong forces, which are called nuclear forces, keep protons and neutrons together (close to each other).
The first successful model for the atom was suggested by Rutherford.
After that, experiments gave data that Rutherford's model could not explain, so Bohr suggested a new model.
More recent experiments gave new data that even the model of Bohr could not explain, thus an even newer model was suggested by Schrodinger.