Insider trading is any trading done on material non-public information relating to an instrument.
If my sister, who works for a drugs testing company, tells me that stage 3 trials of a drug look like they will fail and I trade on that information (probably by shorting a company's stock) that is insider trading. If an employee of that firm trades on that same stock knowing that the trials are likely to fail that is too. If an employee of that company trades on the stock without knowing that information that is NOT insider trading.
If I know from an insider I met at the pub that a large orange producer has seen a fall in production due to a blight and I trade on oranges futures, even though I am not directly trading in the stock it is still insider trading.
I mentioned that the information must be material, that means that it must have the potential to move the market; if I know that a firm is going to increase profits by 10% this year it is not material if analyst expectations are for a similar rise.
You are right that small scale insider trading, such as by employees and their families, is relatively unregulated and unchecked but directors and C-level employees of a firm are required to publish all and any dealings that they have in the stock and several have been caught and penalized for insider trading.
edit: http://www.sec.gov/spotlight/insidertrading/cases.shtml details some cases, many involving director and C-level employees, that the SEC has prosecuted recently.
Incidentally I work in financial fraud monitoring and we use an analytic based on previous days' trades and today's news (i.e. when the information becomes public) to identify traders who might either be indulging in or receiving orders to trade on insider information. Essentially this works by looking for large changes in position against an instrument that later has material information releases relevant to it.
One final thing to think about: given that being caught will generally cause perpetrators to go to prison and be banned from director level jobs and/or trading for life as well as a large, life-changing fine and a massive loss of reputation not many people with insider information want to risk trading on it, myself included.