Hollaback is an international movement against street harrassment. It's composed mainly of a series of websites in different cities and countries where people (mostly women) can post incidents of harrassment, and share how they reacted. Many people also post pictures they took of their harrassers as well.
The first website was started in New York City, now it is international. Any incident of harrassment can be posted. People write about being flashed, being grabbed, bothered, intimidated, insulted, and followed. One of the points of hollaback is to draw attention to the fact that street harrassment happens very frequently, to many, many women, all over the world. Something that happens to the majority of women (half the population) on a daily basis all over the word is worth talking about. In some cultures all women are supposed to feel complimented when strangers compliment her on her appearance, and very likely some women do feel complimented, but many do not, and it is not socially acceptable for them to discuss it or say that they are bothered by it or take it seriously, so hollaback creates a space to do so.
Also, it is a space to share ideas on how to deal with street harrassment, because many women feel that you can't do much when a stranger totally disrespects and enrages you: it's not like the police will give any real response (though you can try your luck with them), and you usually can't beat your harrasser up. What you can do is respond (which is what the website encourages, thus its name), and women share how different responses, from profanity, to polite requests, to well places comebacks, to snapping a harrasser's photo, have worked.
A lot of what is talked about on the website is everyone's right to be able to move through and exist in public space, and how sometimes women, people of color (especially women of color), LGBTQ and disabeled people are made unwelcome in and sometimes even have restricted access to public space, because various strangers feel they have the right to comment on/police our appearance and impose themselves on our lives.
Reading that some other women feel pissed off and/or intimidated by street harrassment was cathartic for me, because I destest it. Sharing instances of street harrassment also makes me feel better and like I don't have to just roll over and take it. The main website is http://www.ihollaback.org/