The confusion between viruses, worms, and trojans matters because understanding the type tells you how an infection spreads and what behavior enables it. A virus needs you to share an infected file — such as emailing a document. A worm needs no human involvement and spreads by exploiting network vulnerabilities automatically.
A worm scans networks for vulnerable systems and installs itself without any user action required. The WannaCry ransomware of 2017 was a worm — it spread to hundreds of thousands of computers worldwide within hours without anyone clicking anything, by exploiting an unpatched Windows vulnerability.
Trojans are the most common type encountered by everyday users because they require no exploit — just deception. The user willingly installs what they believe is legitimate software. This is why download sources matter enormously: software from official stores goes through review; software from random websites does not.
Adware and spyware are usually delivered as trojans, bundled with free software. A free PDF converter or game that also installs tracking software is a trojan by definition. Always read installation screens carefully and download software only from sources you trust.