Drive-by download means three things, each concerning the unintended download of computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 software from the Internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

:

1. Downloads which a person authorized but without understanding the consequences (e.g. downloads which install an unknown or counterfeit executable program, ActiveX
   ActiveX
   ActiveX is a framework for defining reusable software components in a programming language-independent way. Software applications can then be composed from one or more of these components in order to provide their functionality....
   
    component, or Java applet).
2. Any download that happens without a person's knowledge.
3. Download of spyware
   Spyware
   Spyware is a type of malware that can be installed on computers, and which collects small pieces of information about users without their knowledge. The presence of spyware is typically hidden from the user, and can be difficult to detect. Typically, spyware is secretly installed on the user's...
   
   , a computer virus
   Computer virus
   A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...
   
    or any kind of malware
   Malware
   Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...
   
    that happens without a person's knowledge.

Drive-by downloads may happen when visiting a website

Website

A website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...

, viewing an e-mail message or by clicking on a deceptive pop-up window: by clicking on the window in the mistaken belief that, for instance, an error report from the computer itself is being acknowledged, or that an innocuous advertisement pop-up is being dismissed. In such cases, the "supplier" may claim that the person "consented" to the download although actually unaware of having started an unwanted or malicious software download. Websites that exploit the Windows Metafile vulnerability

Windows Metafile vulnerability

The Windows Metafile vulnerability is a security vulnerability in Microsoft Windows NT-based operating systems which has been used in a variety of exploits since late December 2005. The vulnerability was first discussed in the computer security community around 26 and December 27, 2005, with the...

 (eliminated by a Windows update

Windows Update

Windows Update is a service provided by Microsoft that provides updates for the Microsoft Windows operating system and its installed components, including Internet Explorer...

 of 5 January 2006) may provide examples of drive-by downloads of this sort.

A drive-by install (or installation) is a similar event. It refers to installation

Installation (computer programs)

Installation of a program is the act of putting the program onto a computer system so that it can be executed....

rather than download (though sometimes the two terms are used interchangeably).