Defination of Dividend can be found here basic definition.

http://www.investopedia.com/terms/d/dividend.asp

My Kopitiam Explanation is...

Normally given out by long established companies, who continuously / consistently generate substantial profit.

This profits after minus all expenses still got huge surplus so they distribute to share holderslor.

Why distribute to shareholders you ask?

Coz we take risk to investmah and pump the company with money.

Risk = Reward

This is layman defination but got many other reasons why company want to distri dividend.