"Downloading" is the transmission of a file from one computer system or network to another smaller computer system. "Uploading" is transmission in the other direction—from a smaller computer to another larger computer or network.
Many users download files from the Internet or upload files to the Internet. People who share files with others on bulletin board systems (BBS) must upload files to the BBS. File sharing programs like LimeWire and DirectConnect encourage users to make files on their computers available for upload.
In short, to download is to receive a file and to upload is to send a file.
Downloading data from an unknown or unreliable source can be dangerous. Many files available for downloading contain malware. A Trojan horse, for example, is an apparently harmless program that contains malicious or destructive code. Left alone, it has the ability to hurt your computer in a number of ways, such as ruining your hard disk, or sending out your personal information to a hacker.
One of the dangers of downloading from the Internet is spyware. The term refers to any software that aids in the gathering of information about a person or computer without the owner's knowledge or consent. Usually, the information gathered is forwarded to advertisers or other interested parties, sometimes even hackers. Downloaded programs can often contain spyware, as spyware writers use "free" downloads to distribute their product.
Some data collection programs are installed with the user's knowledge; they are not considered spyware if the user fully understands what information is being collected and with whom it is being shared.
The CERT® Coordination Center has important information about downloading and installing new software.
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