Structure of the Earth in Brief

Earth has a radially layered structure, like that of an onion. The major divisions of the earth interior is based on the seismological observations. Earth has mainly three chemically distinct layers that have distinct elastic properties and densities:

1. Crust:- This is the thin and very heterogeneous layer near the surface, which is composed of light silicate minerals such as feldspar and quartz.The seismic structure of the continental crust is variable and it has an average thickness of 35 km. The oceanic crust is thinner, 7-8 km thick. The crust has been derived from mantle by a series of melting and reworking processes. 
2. Mantle:- This layer is composed mainly of denser silicate minerals such as olivine. Its depth continues to about 2900 km. Since, both compressional and shear waves propagate through it, it must be solid to a large extent. 
3. Core:- This layer is considered to be made predominantly of iron with lesser amounts of other elements. The outer core will not transmit S-waves and is interpreted to be liquid. It extends from the 2900-km to the 5200-km discontinuity. The inner core, which extends from the 5200-km discontinuity to the center of the Earth, transmits S-waves—although at very low velocities, suggesting that it is a solid near the melting point.

Mohorovicic Discontinuity: This is the discontinuity between the crust and mantle. It is also referred to as 'Moho'. It separates the rocks having P-wave velocities of 6-7 km/s from those having velocities of about 8 km/s. It ranges in depth from about 3 km at ocean ridges to 70 km in collisional orogens.
The Moho may represent a transition from mafic to ultramafic rocks or to a high pressure assemblage composed predominantly of garnet and clinopyroxene. It is commonly assumed that 'seismic Moho' is also 'Petrological Moho', the boundary between sialic or mafic crustal rocks and the ultramafic mantle rocks.
In some crust, such as collisional orogens, the Moho is often offset by complex thrust faults. The Himalayan orogen is a superb example in which a 20-km offset in the Moho is recognized beneath the Indus suture (Hirn et al.,1984). This offset was produced as crustal slices were thrust on top of each other during the Himalayan collision.

Conrad Discontinuity: In some continental crust, there is evidence of a small discontinuity at midcrustal depths, called the Conrad discontinuity (Litak and Brown, 1989). When identified, the Conrad discontinuity varies in depth and character from region to region, suggesting that, unlike the Moho, it is not a fundamental property of the continental crust and it is diverse in origin.

The Crust, Mantle and Core is the compositional division of the Earth. The Earth is also divided into different layers based on its mechanical strength. These are:

1. Lithosphere:- This is cool and therefore mechanically strong outermost layer of the Earth. It is of the order of 100 km thick (varies from 10-200 km) and comprises crust and uppermost mantle.
2. Asthenosphere:- This layer lies just beneath the lithosphere. Because of high temperature and pressure  at this depth, its viscosity is low enough to allow viscous flow to take place on geological time scale.
3. Mesosphere:- It lies below depth of 670 km. The difference between asthenosphere and mesosphere is due to increased density and rigidity. At the depth of 670 km there is phase transition from spinel --->  oxides, perovskite. 
4. Core: Same as above

Notes:

• If the Earth is viewed in purely mechanical terms, the mechanically strong lithosphere floats on the mechanically weak asthenosphere.
• In lithosphere, the heat is lost by conduction and in asthenosphere, the heat is transferred by convection.
• At depths of 400 and 670 km, there are sharp changes in P and S wave velocity (increases by 5-7%). This entire region is often called mantle transition zone.
• The lower mantle at depths down to 2700 km is referred to as the D' shell. The lowermost 150-200 km of the mantle is referred to as D" layer. The velocity gradients are much reduced in the D" shell. This could be due to chemical heterogeneity and interaction between core and mantle, and/or to a thermal boundary layer. The temperature contrast between mantle and core is 1000 to 500 K
• A region of low seismic-wave velocity and of high attenuation of seismic-wave energy, the low-velocity zone (LVZ), occurs at the top of the asthenosphere and is from 50 to 100 km thick.
• The  upper mantle extends from the Moho to the 660-km discontinuity and includes the lower part of the lithosphere and the upper part of the asthenosphere. The  lower mantle extends from the 660-km to the 2900-km discontinuity at the core–mantle boundary. The lower mantle is also referred to as the mesosphere, a region that is strong but relatively passive in terms of deformational processes.

References:

1. The Solid Earth: An Introduction to Global Geophysics by C M R Fowler
2. The Dynamic Structure of the Deep Earth by Shun-ichiro Karato
3. Fundamentals of Geophysics by William Lowrie
4. New Theory of the Earth by Don L Anderson
5. Earth as an evolving Planetary System by Condie