title: What issue are .NET Standard and .NET Core intended to solve?
body: <p>I've read some things about .NET Standard and .NET Core, and generally they seem to say something like "This is the new way to do things, and here are the advantages and disadvantages."</p>
  
  <p>What I haven't seen is a good, clear explanation of what was wrong with the status quo, such as .NET Standard and .NET Core were even necessarily.  To really judge what I should use these for, it would be helpful to know why they exist at all.</p>
  
  <p>Can someone give a clear (and not necessarily concise!) explanation of (a) what <em>problem</em> .NET Standard and .NET Core are intended to solve, and (b) how they solve it?</p>