What is a virus downloader?

A downloader is a program that automatically downloads and runs and/or installs other software without the user's knowledge or permission. In addition to downloading and installing other software, it may download updated versions of itself. A downloader may install itself in a manner that allows it to constantly check for updated files. For example, it may add an entry to the following registry key: HKLM\Software\Microsoft\Windows\CurrentVersion\Run