All the information that can be found about you on the internet is referred to as your 'online presence'. Increasingly this information comes from your profiles on social networks such as Twitter, Facebook and LinkedIn. It also includes photographs of you and anything that you have written or has been written about you. This may be on blogs, forum posts and wikis.
Don't assume your prospective employers won't Google your name, even if this is not an official part of the selection process. When they do, if the first result is a photo of you half-dressed and falling into a hedge on your birthday, that doesn't immediately convey a measured attitude to life and you're unlikely to get that dream job in risk management. Un-tag yourself in any potentially compromising photos on Facebook and use their 'View As' function to see how your profile looks to the public. Facebook is known for its frequent alteration of privacy settings, so make sure you check your profile often.
If you've applied for a job where you will be expected to communicate with clients or potential customers, it's quite likely that someone from the company will search for you on Twitter. If they scroll through your timeline and your tweets are all in capitals and are mainly sweary, emoticon-ridden rants at judges on The X Factor, they may no longer see you as the composed, well-educated person your CV conveys.
LinkedIn is arguably more difficult to make mistakes with as its purpose is to convert you into an ever-professional living CV. However, just because your photo is in focus and you're not posting offensive updates about previous employers, don't assume you're a LinkedIn success. If the employment history on your LinkedIn profile is different to that on the CV you've been emailing to recruiters or potential employers, it looks suspicious. Poor spelling and grammar, and few connections or endorsements can also create a negative impression.
Before you apply for that job, Google yourself and see what comes up.