If you setup your machine improperly, then yes, someone could see things on your screen. Most notably, having screen sharing turned on, but not protected. This isn't phishing though. Phishing is about going to sites that appear to be a site you know (such as your bank site), but really aren't and just steal your account credentials. Every machine is vulnerable to this because the user has to be aware of where they really are and how they got there. Some browsers do try to keep track of known bad sites, but that only checks against known ones. User vigilance is the best defense against phishing.