Bohr's model of hydrogen atoms is based on following assumptions:

the electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy, these paths are called orbits, stationary states or allowed energy states. These orbits are arranged concentrically around the nucleus
the energy of an electron in the orbit does not change with time, however, the electron will move from a lower stationary state to a higher stationary state when the required amount of energy is absorbed by the electron or the energy is emitted when electron moves from higher energy state to lower energy state. The energy change does not take place in a continuous manner (the frequency of radiation absorbed or emitted is given by: energy difference between two orbits is product of frequency of radiation and planck's constant)
the angular momentum of electron in a given stationary state can be expressed as : mvr=nh/2(pi) (where n=1,2,3...)
thus an electron can move only in those orbits for which the angular momentum is integral multiple of h/2(pi), that's why only fixed orbits are allowed.

Limitations of Bohr's model

It violates the Heisenberg Uncertainty Principle. The Bohr Model considers electrons to have both a known radius and orbit, which is contradictory to the uncertainty principle.
The Bohr Model is very limited in terms of size. that is Bohr model is applicable only to hydrogen or hydrogen like atoms.
It does not explain the Zeeman Effect, when the spectral line is split into several components in the presence of a magnetic field.
The Bohr Model does not account for the fact that accelerating electrons do not emit electromagnetic radiation.

Bohr's Model for Hydrogen Atom