using-directives

ASP.net c#, how do I know what to use?

I keep coming accross code samples online for ASP.net c#, however they never seem to list which namespaces they include, for example: using System.Data.SqlClient; etc etc Am I missing something obvious or should I be expected to know exactly what namespaces each code example requires? ...

What does the using directive do,exactly?

Hi, On MSDN I can read what it does, but I would like to know what it does technically (tells compiler where to look for types..)? I mean using as a directive. ...