A voyeur is "the one who looks", the one who loves observing people, mostly without them being aware of it.
Most of us aren't aware that we're also being watched on the internet. Our privacy is harmed by companies who want to analyze our surfing behaviour and use the information afterwards to persuade us with personalized advertisements or adapted prices. Also others like to spy on the things we're trying to find on the internet, but i'm not planning on digressing the subject.
The world wide web is keeping track of your every move. The information per IP-adress, or in some cases even per computer is being sold to third parties. You may have noticed that in some online web stores you'll pay €550 for that TV while a friend is aloud to buy that same device for only €500. This is a new tactic that online web stores are using more frequently nowadays. The prices are adapted to the online buying behaviour of the clients. If it appears that a client doesn't look at a euro more or less: why don't increase the price? It's completely legal.
Giants such as Facebook and Google hold a lot of information that we may not even want them to have. If you're planning to go on a vacation and look up some information about the desired travel destination, it's very likely that some advertisements of cheap flights and other sunny destinations will appear on the sides of your screen. In some cases it even goes so far that after you've booked a trip, the prices of a bikini in the web store will increase because they are sure that you're planning on buying one.
This subject reminds me of Febelfin, the Belgian federation of the financial sector, who came up with a great campaign a few years ago to warn people to be careful with what you throw on the internet. Complete strangers can find out everything about you if they want to. It's really kind of creepy.