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a good starting point :

http://en.wikipedia.org/wiki/Electronic_band_structure

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then :

http://web.mit.edu/course/6/6.732/www/new_part1.pdf

and the references therein

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See electronic band structure of semiconductors.............

Please see

http://www-rjn.physics.ox.ac.uk/lectures/semiconnotes10.pdf

http://www.doitpoms.ac.uk/tlplib/semiconductors/junction_rectifying.php

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html

I didn't get u exactly, but the dispersion in the bands (either thy are flat or steeper is completely depend on, to what an extent they are hybridized or overlapped with the neighboring atomic orbitals. the flat band represents a poor overlap with the other atomic orbitals and a steeper band represents the compleete overlap with the other atomic orbitals. for example, in case of graphene one 2pz orbital of one C atome overlapped with the other 2pz orbital of another C atom gives a liner shape to the bands). I think it may not completely clarify your doubt but give whole idea about the band shapes in a band structure  

thanks to all

Hi, I think everyone gave good and correct explanations, however, I like to describe it in a way we can have a deeper insight. The energy bands is a nice way to show how an electron see  the "world" around it!  E-k space is the the "world" an electron can only see so when it moves in material made up of atoms this space becomes very different world compared to to if there was no other atom exist. So depending upon the material (metal, semiconductor or insulator) electron movement is constrained by the atomic arrangement of the material hence the energy bands allowed for the movement of electrons. Energy bands in metals are over-lapped hence conduction is possible without restriction while in semiconductor by nature of material there are gaps in energy bands and free electrons can move in conduction bands. Again the distribution type of atomic arrangement ( i.e. their bonding nature and type etc.) the conduction may offer difference in terms of ease for electrons to move in certain direction hence X L and gamma . So an electron would "see" that  it can move in a direction that is most easy unless it is pumped by a mechanism to placed in an other direction where it may or may not see the ease for its motion. So if an electron moving in one direction and through the application of external force (for example electrical stimulation) it might be placed in a direction where its motion is not that much at ease. It would, then as a result, slow down as if its mass has increased. The ease and difficulty of each of these directions is represented by the curvature of the energy bands offered to electrons in the material structure. Therefore,  slowing down of an electron would be a natural phenomena due to material and such an effect called Gunn effect was  discovered over 50 year ago can easily be explained on the basis of this above explanation.

I hope this would be useful when you read the full theoretical and mathematical description of energy bands.

To Add to Ata H Khalid's answer, energy bands are not of valley shape, the shape is just to represent the electron energy and therefore the valleys in the energy bands indicate the points where electron has minimum energy. 

Thank you Ata H Khalid

you give me a nice picture about the valley of CB and VB.