Malware is commonly delivered as a malicious attachment or link in a phishing email. Most malware email attachments include code or exploits which cause your computer to download more malware from the Internet. Some malicious programs can be delivered via a USB drive.
Most households will be familiar with promotional material for everything from the current bargains at the local supermarket to what's on the local theatre, delivered to you through your letterbox.
The electronic version is called junk/spam email which promotes everything from… well just about everything!