How about reference type?

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It will show the reference page when I right click a web project and select
it's property page.
There are some reference types: BIN, GAC
What is the difference between these two types?

 

[Marc Gravell]

BIN references are where you are referencing the binary directly; the
dll typically gets copied into the ~/bin folder of the app. A GAC
reference obtains the dll from the Global Assembly Cache; the dll is not
required separately in the application, but the dll must have been
previously installed into the GAC by an install step.

It is much easier to deploy BIN references, since this is just a file
copy (simply use robocopy etc). GAC references are useful for things
that don't change as much - core .NET dlls, for example, often (but not
always) get installed into the GAC. For web sites, I try to avoid the
GAC... it makes deployment unnecessarily complex...

Marc