Mal-what?

Malware.

It's the "stuff" that those who help others maintain their computers have nightmares about.

To many, talking about "malware" seems like speaking a new language. But, here's a nice summary:

Malware, short for malicious software, is any software used to disrupt computer operations, gather sensitive information, gain access to private computer systems, or display unwanted advertising. Before the term malware was coined by Yisrael Radai in 1990, malicious software was referred to as computer viruses   The first category of malware propagation concerns parasitic software fragments that attach themselves to some existing executable content. The fragment may be machine code that infects some existing application, utility, or system program, or even the code used to boot a computer system.

Malware is defined by its malicious intent, acting against the requirements of the computer user, and does not include software that causes unintentional harm due to some deficiency.

Scareware is a form of malicious software that uses social engineering to cause shock, anxiety, or the perception of a threat in order to manipulate users into buying unwanted software. Scareware is part of a class of malicious software that includes rogue security software, ransomware and other scam software with their computer, then suggesting that they download and pay for fake antivirus software to remove it.  Usually, the virus is fictional and the software is non-functional or malware itself.

The "You've got 6,000 files cluttering your computer. Click HERE to fix them!" messages are in this category.

Ransomware is computer malware that installs covertly on a victim's computer, executes a cryptovirology attack that adversely affects it, and demands a ransom payment to restore it. Simple ransomware may lock the system in a way which is not difficult for a knowledgeable person to reverse, and display a message requesting payment to unlock it. More advanced malware encrypts the victim's files, making them inaccessible, and demands a ransom payment to decrypt them.  The ransomware may also encrypt the computer's Master File Table (MFT) or the entire hard drive.  Thus, ransomware is a denial-of-access attack that prevents computer users from accessing files since it is intractable to decrypt the files without the decryption key. Ransomware attacks are typically carried out using a Trojan that has a payload disguised as a legitimate file.

These are a bigger risk to the data of organizations and businesses than they are to common people, because although your photos of Uncle Leroy's 89th Birthday Party probably are not replaceable, you are unlikely to be willing to fork over $1,000 or more for getting them back... 

Spyware is software that aims to gather information about a person or organization without their knowledge and that may send such information to another entity without the consumer's consent, or that asserts control over a computer without the consumer's knowledge.

"Spyware" is mostly classified into four types: system monitors, trojans, adware and tracking cookies.  Spyware is mostly used for the purposes of tracking and storing Internet users' movements on the Web and serving up pop-up ads to Internet users.

Whenever spyware is used for malicious purposes, its presence is typically hidden from the user and can be difficult to detect. Some spyware, such as keyloggers, may be installed by the owner of a shared, corporate, or public computer intentionally in order to monitor users. This is one reason why it is best to assume that nothing, and I mean, NOTHING you do or view or type on the computer is entirely secret.

Yes, your financial information for your bank is encrypted, so probably quite safe. But, consider the forensic persona evidenced by your Facebook Profile!