Exactly what is 'Thinking Outside the Box'?
when we say some one is thinking 'outside the box'? why do we need this 'think out of box thing?'

lets consider this scenario:
There was a company hiring new staff and 200 candidates were short listed and called for interview.
They were given a situation where they have to make a choice and justify why they made that choice.
It goes like this:

One day, you are driving home and passed a bus stop where 3 persons were waiting for bus. The first person is a very sick old lady who need to go to the hospital a.s.a.p. The second person is a friend of you who once saved you life, you owe him 1 and now he/she need your help desperately. And the third person happened to be the dream person of yours, she/he can be your ideal partner! But unfortunately, your car can only accommodate one single person. You wanted to help them, yet there is only 1 person can be helped. Who will you choose? and why?

All the candidates tried hard to answer this question and they had given their justification who will they choose. It was a very hard situation, if you decided to help the old lady, what might your life saver think about you? He/she needs your help too! And if you help the old lady or your friend, you might loose the once in a life time chance to get you dream partner!
While all the candidates made their choice and justified about their choice, there was one person who answered differently from the rest, and he was hired to the position offered.
His answer was like this:

"I will give my car to my life saver and request him/her to send the sick old lady to the hospital on the way, and i'll stay at the bus stop with my dream person so i can spend some time with her"

While everyone was thinking how to make the choice, this guy had the most innovative idea on how to solve the problem. While everyone's mind was restricted with the rule "i can only help one person and i need to choose one", this guy had thought out of that restriction and context. Well, we can say he was the person who "think out of box".

Inside The Box
Thinking inside the box means accepting the status quo.
The in-the-box thinkers also believe that every problem needs only one solution; therefore, finding more than one possible solution is a waste of time. They often say, "There is no time for creative solutions. We just need THE solution." But, they never realize that there are other different ways of solving the problem.

Outside the Box
Thinking outside the box requires different attributes that include:

• Willingness to take new perspectives to day-to-day work.
• Openness to do different things and to do things differently.
• Focusing on the value of finding new ideas and acting on them.
• Striving to create value in new ways.
• Listening to others.
• Supporting and respecting others when they come up with new ideas.

Out-of-the box thinking requires an openness to new ways of seeing the world and a willingness to explore. Out-of-the box thinkers know that new ideas need nurturing and support. They also know that having an idea is good but acting on it is more important. Results are what count. ^[1]

"Thinking out of box" means think out of the context of the situation we are facing. Don't be restricted by the situation when searching for a solution. Normally when it comes to decision making and making choices, instead of thinking which one to pick, why don't we think another way so that we will not be bonded by the choices we have?

There's once my friends and i were discussing about fairy tales, and someone asked:
"If a fairy appear in front of you and agree to grant you 3 wishes, what will you wish for?"
Everyone told what they will wish for. Well, its hard to think only 3 wishes that are most important for us! When it was my turn, I said:" I doesn't need 3 wishes from the fairy, but just one will be enough. I wish i will have endless wishes!....."

References:
[1] - Exactly what is 'Thinking Outside the Box'? By Ed Bernacki