What is the difference (in terms of use) between namespaces in C# and packages in Java?
There are a few details that differ.
In Java the directory structure should match the package structure. No such restriction in C#.
In C# you can have multiple namespaces in one file. In Java one file belongs to one package (see previous).
Java has default/package accessibility. C# internal accessibility goes in assemblies.
If you use VS and Eclipse and let them structure the project, then you will not feel the differences much.