SharePoint is a massive "application platform" - an intranet portal, an external website, a collaboration tool, a business intelligence agent, a content management system, a document storage system, etc. that uses ASP.NET . (SP comes with hundreds, if not thousands, of ASPX pages.)
On the other hand, for a simple .NET site, you do not need to install SP: only IIS with any required ASP.NET extensions.
You use SP when you need it and / or are encouraged to use it. This is good - but usually not teriffic - despite what it does (and that's a lot). But he is big and expensive. (Microsoft recommends "something like at least 7 servers for the" main "SP farm, but can work as a single instance.)
What is it. Actually.
As others noted, the SharePoint Foundation is โfreeโ because there is no SharePoint CAL or SharePoint card. However, do not reduce the cost of any additional requirements.
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