A website or web service may be unreachable from your location as a result of Internet filtering (censorship). Access may be restricted from your network, service provider or even from your country. The requested resource may have been knocked offline by a malicious cyber-attack. Read more about Internet censorship on Wikipedia.
Self explanatory. You can lose your digital data by accident, misfortune or as a result of malicious action.
Since we cannot usually see or hear the person sending us an email or a chat message – we assume their identity by their email address or account nick name. However these identifiers are easy to fake on the Internet.
Data retention legislation and Surveillance often lead to the creation of a profile of your actions. These profiles can be used to make assumptions about your identity and future actions, as well as sold to marketing agencies and data brokers. Browser cookies, when set and queried for by various websites can also result in a dossier of your browsing preferences and general interests. The process of gathering information to identify your habits and friends (social graph) on the Internet is referred to as profiling.
Refers to the monitoring of your online activity at any intersection between your device and the website you are visiting, email account you are writing to, person you are speaking to. For more info look to the various guides listed in the FAQ or in Wikipedia
Access to a protected resource without permission is often times unauthorized and may be malicious. This process and its repercussions are very well described in this Wired article. There are many ways in which someone may gain unauthorized access to your accounts or devices including: cracking your password through profiling or brute-force attacks, social engineering, installing malware on your device, etc.