Security both personal and corporate, is crucial in many applications. In simple applications this might simply involve a login screen, but in high stakes situations, such as banking, more complex authentication mechanisms may be needed.
However, security is not simply about authentication: one might need to consider different kinds of attacks (remote internet access or a colleague in the same office), accidental disclosures (e.g. mailing a spreadsheet with hidden confidential columns), and physical security such as leaving a laptop on the train.
Security is closely connected with privacy and safety, if a system is insecure it will often be possible to access confidential information or affect critical systems. Very often major security breaches involve human-oriented attacks, such as phishing.