The term 'malware' refers to a broad category of software that is designed to infiltrate or damage a system without a user's consent (e.g. malicious software). This category includes more specific types of malicious software, such as spyware and adware, which are both types of malware designed for a specific purpose such as generating advertising revenue. Typical functions of malware include:

• Collecting personal information (email addresses, banking details, etc)
• Redirecting your web browser to a different web page
• Inserting advertisements into web pages that you visit

Malware is typically installed without the knowledge of the user and is notoriously difficult to remove. It also often operates behind the scenes without any obvious signs of activity and is a common cause of problems such as visiting websites, computer slowness, program crashes, etc.