Involves trading of stocks and securities by a group or individuals who might have access to non-publicly available information which could affect the price of the company.
"Insider trading" is a term subject to many definitions and connotations and it encompasses both legal and prohibited activity. Insider trading takes place legally every day, when corporate insiders – officers, directors or employees – buy or sell stock in their own companies within the confines of company policy and the regulations governing this trading. 
Company executives “insiders,” are allowed to buy stocks in there own companies, with there own money, for there own accounts, as much as they want, as long as they declare their transactions publicly. Insiders have the most knowledge of the inner workings and future prospects of their company. There is undoubtedly plenty of important, non-public information influencing insiders’ investment decisions regarding their own firms’ shares.
Sometimes called the “Secret Hobby,” of executives on Wall Street. They have nothing better to do with their lives than figure out where their own company’s stock is headed, and then act on that information for their personal accounts. Insider’s may imagine a big event coming, maybe a few times in their working career. If they can get a pile of liquid capital together, they can trade in there own firms securities.
Although they cannot sell for at least six months, changes in ownership must be received at the SEC by the second business day following the trade, to let investors know how insiders are benefiting from the unfair advantage they have when trading in there own company’s shares.
"Individual investors think – and I passionately believe – that the proverbial little guy' on Main Street should have the same fair chance as the big guys.' 
It is the trading that takes place when those privileged with confidential information about important events use the special advantage of that knowledge to reap profits or avoid losses on the stock market, to the detriment of the source of the information and to the typical investors who buy or sell their stock without the advantage of "inside" information.
Some argue that insider trading is a legitimate form of compensation for corporate employees, permitting lower salaries that, in turn, benefits shareholders. 
insider "one in possession of special information by virtue of being within some organization," 1848, from inside + -er (1). Originally in reference to the stock markets. 
trade (n.) 14c., "path, track, course of action," introduced by the Hanse merchants, trade "track, course" (probably originally of a trading ship). Sense of "one's habitual business" (1540s) developed from the notion of "way, course, manner of life" (mid-15c.); sense of "buying and selling" is first recorded 1550s. Trade wind (1640s) has nothing to do with commerce, but preserves the obsolete sense of "in a habitual or regular course." Trade union is attested from 1831.