The term harmful software refers to software and programs that include, among other:

• Viruses
• Worms
• Trojan horses
• Spyware

Programs that are designed to "intrude" into our computer and create unwanted side effects, such as weakness of the computer, improper functioning applications or sudden appearance of various characters and numbers on the screen. But a virus can cause serious and irreparable damage (such as the destruction of files from the hard disk).

Worm programs are viruses that reproduce by making copies of themselves through computer networks. A worm can harm a network and can greatly decrease the speed of the Internet consume all the resources of the computer and even lead to a shutdown.

Programs that seem to work correctly and lawfully, in fact can cause several injuries. So named because (like the Trojan Horse of Troy) can enter into our computer through processes that are considered harmless, for example through a game or via a virus scanning program, and so deceive us hiding backstage activity.
Generally Trojan horses can not multiply as viruses. A Trojan horse can act as a spy, that start to record anything typed on the keyboard us. This will collect information passwords, accounts, banking transactions and possible other personal data.

Programs that secretly attach themselves to files you download from the Internet. Once downloaded, aftoegkathistatai our computer and starts the monitoring of our web activity. The information is then sent to record the third, which mostly are companies interested in creating our own profile and start sending our promotional or other material depending on our interests.
The spyware can also gather information about e-mail, passwords and even credit card numbers.