Sometimes these things are harmless and just make your computer become a total pain to work on, other times your computer could have some very serious issues that can cause complete data loss. Most commonly your computer will be infected with malware, adware and viruses.
A virus is a piece of code or a program that is installed onto your computer without user interaction. This software will run without your permission and is usually set to start automatically.
Viruses have the ability to consistently duplicate themselves which will decrease system performance with every copy.
A worm is defined as a self-replicating malicious program that utilizes networks to transfer copies of itself to other devices on the network. This can cause harm by spiking bandwidth usage, attaching itself to e-mail attachments, shared network files and can also corrupt data.
Trojans are malicious software or scripts that do not duplicate/replicate themselves but have a different agenda all together. When trojans are activated either by user interaction or a startup script. They start to carry out the script that the programmer wanted the trojan to carry out. A trojan can unlock your computers security so that remote connections can be initiated to your computer. Trojans are most famous for data loss, system corruption and data theft.
Malware is an abbreviation for malicious software. Any software that will slow or disable computer operation, collects personal data or can create access to computers and their networks.
Spyware works in the shadows. Spyware can gather information about computer activities and can gather and collect personal data as well. It is capable of searching and scraping hard drives for data to send back to the programmer.
The most annoying of them all! Adware obviously displays ads, it usually redirects your home page to an ad littered search engine and will display ads all over your computer and browser. Adware usually redirects all your traffic through a proxy server where personal data can be obtained.