Four realms of Mechanics

1.Classical Mechanics: When we talk of macroscopic objects we are actually dealing with classical mechanics. Here the speed (v) of objects considered is small as compared to velocity of light (c), i.e. v <<< c.

2.Quantum Mechanics: For the microscopic objects (of the order of size of an atom) we deal with quantum mechanics.

3.Relativistic Mechanics: For objects that travel very fast (at speeds comparable to c), classical mechanics transforms into relativistic mechanics.

4.Quantum Field Theory: For objects which are microscopic as well as travel with the speed of light, we require the theory which incorporates relativity and quantum principles and thereby we have theory entitled 'The Quantum Field Theory'.

Source:Introduction to elementary particles by David Griffiths