Let's start at the beginning

Simple deploy description

I have a bunch of posts that relate to deploying a webpart.
Building a webpart is not real tricky - it's a standard ".net"
dll that inherits form System.Web.UI.WebControls.WebParts or alternatively - Microsoft.SharePoint.WebPartPages.WebPart which are related classes.

So if you're old enough to remember cgi - that means
at one time interactive web pages were done with a perl
script that was connected to the server via a real simple
mechanism --- there was a directory it went into and the server
dealt with the issue of connecting your perl script to
the html. Of course that was then and now is now.

Here is another link by Sahil Malik for deployment.

Now, a quick view of these two posts will let you realize that they're putting
the DLL into two different directories. Actually there are at least
three different areas I've seen referenced to put the dll - and then
of course you can "package" your solution in different ways. I'm
guessing they wanted to be flexible with their solutions here.....

So --- _app_bin in the inetpub directory --- bin in the sharepoint
directory or gac ????? and why is that ????? this is a working mystery ---
that I very much would like to have an answer to. Nevertheless, I will
keep digging and if anybody is holding their breath ---- ( maybe you
shouldn't ). My guess is somebody on the sharepoint dev team knows
why. Really this should not be a cause for anxiety. This stuff does
work ---- mostly. Sometimes it's not really important that you know
why.... although I should mention that most of the time I let
myself get along without that bit of info - and finds that it will
come back to bite me - any guess on where?