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Some people protest by taking to the streets, but today, thousands more are doing it while sitting at their computers.

In the past week, protesters have managed to take down websites owned by MasterCard, Visa, PayPal and PostFinance for their actions against WikiLeaks (a website known for publishing controversial and confidential information). Many others have been hit over the past decade, including the governments of Georgia and Burma. Protest shades easily into cyberwarfare, and in the UK this can amount to law breaking behaviour.

A website is easy to attack using a Distributed Denial of Service attack or DDoS. The idea is to send it more traffic than it can handle. The website gets so bogged down processing fake traffic that it doesn't have time to respond to real users, who are therefore deprived of their service. You could get the same effect by having hundreds of people phone the same restaurant to order a pizza, then hang up if they get through.

Websites often get swamped even without being attacked. Servers can crash if too many people want to check their swine flu symptoms or visit online library Europeana, or whatever, and they all try to do it at the same time. The idea behind a DDoS attack is to do it deliberately.

Criminals do it by using networks of remotely-controlled home PCs that have been compromised by computer viruses ("botnets"). The amorphous and unorganised pressure group that calls itself ‘Anonymous’ is doing it mainly by publicising its plans via chatrooms and social networks.

Anonymous has had lots of media coverage because of its association with WikiLeaks (the two are not actually connected), but DDoS attacks have been growing recently. In 2008, Anonymous was involved in DDoS attacks against Scientology.

Then came Operation Payback, which was a response to moves against file-sharing sites such as Pirate Bay: hence Operation Payback's use of a sailing ship logo. This attacked a number of pro-copyright bodies including the UK's Intellectual Property Office (aka Patent Office), and some law firms.

But it's important to note that, in the UK, DDoS attacks are against the law. Following a court case in 2005, sections of the Police and Justice Act 2006 were drafted specifically to tackle DDoS attacks, says Lawdit, adding: "A conviction under this section can result in ten years imprisonment and a fine of £5,000."

If you want to protest online lawfully, you can sign petitions, join Facebook groups, or use Tweetminster to find your MP on Twitter. If you can summon the energy to put pen to paper the old fashioned way, that's even better.

Jack Schofield is a technology journalist and blogger who covered IT for the Guardian from 1983 to 2010. Before specialising in computing, he edited a number of photography magazines and books.