NEW YORK (Fortune) — For Jonathan and Michelle Opp of Chapel Hill, N.C., the Internet, like electricity and indoor plumbing, is an indispensable part of their lives. Always armed with their iPhones, they regularly check travel information and weather forecasts, and even use their devices to find answers to offbeat questions. But there are also major differences in the way the married couple use their devices and Internet connections.

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“Michelle probably does more functional things like shopping or paying bills. I like to spend more of my spare time reading music reviews and checking soccer scores,” says Jonathan, a marketing communications manager.

In fact gender, more so than race, ethnicity or economic status, determines how and what we peruse online. According to a recent study by eMarketer, slightly more women say they use the Internet than men. However once logged on, male Internet users tend to spend more time surfing the Web than females.

Meanwhile, in a separate report, eMarketer estimates that U.S. marketers will spend 37.2 billion dollars on online advertising by the year 2013. Clearly understanding what gets the genders ticking makes economic sense for any business buying ad space on the Web.

Internet Protocol addresses, however, don’t come in shades of pink and blue. So companies eager to reach men tend to focus ads on sports, technology and news sites. Businesses concentrating on women often center on stereotypically female-oriented sites, like parenting Web sites.

“Smart companies use behavioral targeting to try to reach their desired target demo online, but even then, they can’t tell who exactly is behind the IP addresses they are following,” says Lisa Phillips, an eMarketer senior analyst and author of the report “Men Online.”

So what, businesses may ask, is keeping the genders glued to their computer screens? For one, men are much more interested in watching online videos than women, notes Phillips.

The presumption that online images are more appealing to males should hardly come as a surprise: men have long been touted as the more visual sex. Other gender stereotypes seem to carry over to the online world as well: Women, who are often seen as caretakers of a family, tend to click on health care Web sites more frequently than men do.

However companies should be aware that not all Internet tendencies mirror offline generalizations.

“I would say for every situation where you think a trend may be confirming a stereotype, there seems to be another counterintuitive trend that might emerge as well,” says Mary Madden, a senior research specialist at the Pew Internet & American Life Project.

For example, women are often dubbed the more verbally adept sex. However they are no more likely to use online communication tools like e-mail, blogging, or social networks than men are.

And although women are sometimes pegged as more avid shoppers, men are just as keen as women to make online purchases. But their shopping behavior may differ.

“Men generally have the attitude, I’m going to go there, I’ve got to get it and get out,” says Phillips. “Females like to go online and socialize and shop around – much like going into a store.”

Furthermore, Phillips says fathers are just as voracious as mothers about finding online information to improve their children’s health or education. Like Web-savvy moms, they also tend to buy products with their families in mind.

Companies should also be wary about making generalizations on how the genders manage their finances. For years, men have been considered financial authorities in many families. But nowadays women are just as likely as men to bank online, according to the Pew Internet & American Life Project.

And though men are more likely to search the Internet for stock quotes or mortgage interest rates, Phillips says the dwindling economy has more women visiting online job sites. This is despite the fact that men have been hit harder by rising unemployment.

Meanwhile Michelle Opp, a software developer, has no problem admitting she shops online more frequently than her husband. But she insists it has nothing to do with gender.