Server.MapPath

Server.MapPath specifies the relative or virtual path to map to a physical directory.

Server.MapPath(".")1 returns the current physical directory of the file (e.g. aspx) being executed
Server.MapPath("..") returns the parent directory
Server.MapPath("~") returns the physical path to the root of the application
Server.MapPath("/") returns the physical path to the root of the domain name (is not necessarily the same as the root of the application)
An example:

Let's say you pointed a web site application (http://www.example.com/) to

C:\Inetpub\wwwroot
and installed your shop application (sub web as virtual directory in IIS, marked as application) in

D:\WebApps\shop
For example, if you call Server.MapPath in following request:

http://www.example.com/shop/products/GetProduct.aspx?id=2342
then:

Server.MapPath(".")1 returns D:\WebApps\shop\products
Server.MapPath("..") returns D:\WebApps\shop
Server.MapPath("~") returns D:\WebApps\shop
Server.MapPath("/") returns C:\Inetpub\wwwroot
Server.MapPath("/shop") returns D:\WebApps\shop
If Path starts with either a forward (/) or backward slash (\), the MapPath method returns a path as if Path were a full, virtual path.

If Path doesn't start with a slash, the MapPath method returns a path relative to the directory of the request being processed.

Note: in C#, @ is the verbatim literal string operator meaning that the string should be used "as is" and not be processed for escape sequences.

Footnotes

Server.MapPath(null) and Server.MapPath("") will produce this effect too.