<t>AppDomain.CurrentDomain.BaseDirectory is probably the most useful for accessing files whose location is relative to the application install directory.<br/>
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In an ASP.NET application, this will be the application root directory, not the bin subfolder - which is probably what you usually want. In a client application, it will be the directory containing the main executable.<br/>
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In a VSTO 2005 application, it will be the directory containing the VSTO managed assemblies for your application, not, say, the path to the Excel executable.<br/>
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The others may return different directories depending on your environment - for example see @Vimvq1987's answer.<br/>
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CodeBase is the place where a file was found and can be a URL beginning with http://. In which case Location will probably be the assembly download cache. CodeBase is not guaranteed to be set for assemblies in the GAC.<br/>
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UPDATE<br/>
These days (.NET Core, .NET Standard 1.3+ or .NET Framework 4.6+) it's better to use AppContext.BaseDirectory rather than AppDomain.CurrentDomain.BaseDirectory. Both are equivalent, but multiple AppDomains are no longer supported.</t>
