what is the concept behind the selection of spin of electron as +1/2 and -1/2?

Answer 1

You can read about it here. It's a quantity that comes out of particle physics that acts like the particle is spinning even though it can't. The one half means that you have to rotate the particle 360 degrees divided by 1/2 or 720 degrees to get it back to where it started. + and minus just refer to the direction of the spin.

http://en.wikipedia.org/wiki/Spin_(physics)

Answer 2

every atom have orbitals & every orbital have sheel &shubshel .
the subsheel devide in to four parts s,p,d&f. one orbital have max. no. of electron is 2(hund law) . no two electrons in atom can have same value for all quntam no(pauli exlusion law). spin quantm no. decide the drection of electron. in clock wise driction it,s value is +1/2&anti clock wise driction it,s value is -1/2.

Answer 3

it is just a convention
you know that an electron can have two spin states
up or down they called it +,-
just like they are two types of charges you call +.-