For the past 25 years， the Web has opened up unprecedented possibilities for human communication. But it has a dark side too -- sometimes， a very dark one.

Four out of 10 of Web users have been harassed online， according to a survey released Wednesday by the Pew Research Center. And about 73% of the Web users surveyed said they've witnessed someone being harassed online.

The harshest forms of abuse -- physical threats， stalking and sexual harassment -- are often reserved for women.

With responses from 2，849 Web users， the Pew study appears to be the most detailed of its kind looking at abusive behavior on the Web. And it comes at a time when high profile cases of online abuse， many of them targeting women， have bubbled up from the Web's darker corners into the full light of public scrutiny.

\"There's a pretty broad spectrum of these actions，\" said Lee Rainie， director of Internet， Science and Technology research at the center. \"And some of it is quite severe.\"

Online harassment breaks down into two categories， according to Pew. One is minor and one more severe.

The first includes things like name-calling and humiliation. The second category targets a smaller segment of Web users， and includes actions like stalking， harassment over a long period of time and violent threats.

It's that sort of abuse that has made headlines in recent months.

Most prevalent has been GamerGate， a movement that purports to be about defending video-gaming culture and demanding responsible journalism from gaming websites. But it quickly turned into a wave of vicious online attacks on journalists， game developers and others -- almost all of them women.

Several women have been subjected to violent rape threats， detailed death threats， hacking and \"doxxing，\" in which documents with personal information are stolen and posted online. At least two women have been forced to leave their homes after receiving specific threats of violence as a result of GamerGate.

In other high-profile incidents， actor Zelda Williams left social media for several weeks in August after she received graphic， abusive messages on Instagram and Twitter about the death of her father， comedy legend Robin Williams.

And， earlier this month， actor Milla Jovovich announced on Twitter that she'd stopped interacting with fans because she and her family had been threatened by a stalker from the site.

\"After contacting authorities， I was told to absolutely stop talking to people I don't know on social forums，\" she wrote. \"Now that sucks for all the cool people I got to know on here， but my family comes first and I cannot take any chances when it comes to them.\"

It's that sort of intense abuse that concerns Sara Baker， coordinator of Take Back the Tech， a campaign aimed at ending online violence against women.

\"In our work， we see how the effects of online violence play out in the physical world all the time，\" she said. \"Whether or not such abuse results in physical violence -- and it sometimes does -- it always results in real harm.\"

According to the Pew study， women experience the more severe types of abuse at disproportionately high levels when compared to men. Among women age 18-24， 26% said they'd been stalked online and 25% said they'd been sexually harassed online.

Sponsored by members of women's groups from around the world， Baker's organization recently gave a failing grade to Facebook， Twitter and YouTube in a report saying the Web's largest social-media sites don't do enough to prevent abuse of women on their networks.

The Pew report suggests that social media is， in fact， where a lot of online harassment takes place.

Two out of three respondents who said they'd been harassed online said the most recent incident happened on a social-media website or app.

Other sites where people said they encountered harassment included the comments section of websites （22%）， online gaming （16%）， in email （16%）， on a discussion site such as Reddit （10%） and on online dating sites or apps （6%）.

Part of what makes online abuse so rampant is the anonymous nature of some Web forums. Half the people who said they've been harassed online said they didn't know their attacker.

But there's good news： Just as the Web can be used to harass people， it can be used to rally people against hurtful speech as well.

\"The Internet has expanded our ability to respond to such violence by finding community， exchanging knowledge and building solidarity，\" Baker said.

http：//www.edition.cnn.com/2014/10/22/tech/web/trolling-online-abuse/index.html？hpt=te_t1