An atom is the smallest component constituting a chemical element.
Atoms comprise (up to) three different particles:
In the center is the nucleus, consisting of a number of positively charged protons, and a roughly similar number of neutral neutrons.
Around the nucleus is (usually) a cloud of electrons, which can be thought of as orbiting the nucleus like planets around a star.
The number of protons specifies which element it is; for instance, hydrogen has one proton, oxygen has eight protons, and uranium has 92 protons.