Classical mechanics refers to the classical (i.e., non-relativistic, non-quantum) study of physics. Three major formulations of classical mechanics are newtonian mechanics, lagrangian mechanics, and hamiltonian mechanics. The latter two are rather useful in extensions to Classical Mechanics; Special and General Relativity, Quantum Mechanics, and beyond. Rotational dynamics, Statistical Mechanics, and Fluid Mechanics are subsets of Classical Mechanics.
When to Use this Tag
Use classical-mechanics when discussing general concepts of classical mechanics, i.e. the behaviour of macroscopic bodies under the influence of forces (without necessarily specifying the origin of these forces).
Use this tag only if newtonian-mechanics, lagrangian-mechanics, hamiltonian-mechanics, fluid-dynamics, statistical-mechanics, rotational-dynamics and the like are too specific. In general, you should not use classical-mechanics together with general-relativity or quantum-mechanics.
Classical mechanics is the study of the movement of bodies under the influence of forces. In absence of either movement or forces, the subtopics statics and kinematics arise, whereas the ‘complete’ subject is often dubbed dynamics.
For point particles/bodies, there are three equivalent approaches to deriving the trajectories of said bodies: newtonian-mechanics based on Newton’s Laws, lagrangian-mechanics based on the variational principle and hamiltonian-mechanics following from Legendre transformations of Lagrangian mechanics.
More advanced subtopics are fluid-dynamics for the study of moving many-body fluids (liquids, gases), statistical-mechanics for the derivation of macroscopic laws from microscopic princples (often making use of the Hamiltonian formalism) and rotational-dynamics for the study of rotating solid bodies.
Structure and Interpretation of Classical Mechanics, by Sussman, Wisdom, Mayer (available as free ebook at MIT press).
Physics for Scientists and Engineers with Modern Physics by Jewett, and Serway.