Double entry system is the system of book keeping which emerged as a better substitute for single entry system..well there is no any method known as single entry system as per accounting standards...thus not to maintain or improper maintenance of books of accounts is single entry system.
double entry system of bookkeeping is dat system in which the effects of each transaction is posted twice...i.e.  in the ledgers of both the effected accounts..
say if Lokesh has given cash to pankaj..it has two effects...lokesh is giving cash and pankaj is receiving cash...
thus entry passed in lokesh's books of accounts will be
pankaj a/c.....dr
To cash a/c
 
here the transaction is affecting two sides i.e pankaj a/c and cash a/c..

thus pankaj's a/c will be debited with cash (amount what lokesh has given)
and cash a/c will be credited with pankaj a/c with the same amount...
this is how double entry system works...

As per accounting standards...double entry system works on the principle of Dual Effect