Linux is a computer operating system like Windows is an operating system like Mac OSX is an operating system. There are other Operating systems available as well.
Linux is actually the kernel or heart, so to speak, of what is known as a Linux Distribution like Ubuntu, Mint, RedHat, Slackware, etc.
A Linux Distribution will include a selection of desktop GUIs, various applications for browsing, email, word processing, spreadsheets, image manipulation,playing music and movies, etc,etc.
Software is added or updated from the Distributions software repositories via the Distribution's software package manager rather than downloading particular packages from the Software manufacturer's web site.
If you're coming from a Windows background it may be useful to read this: Linux is NOT WIndows
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