FileStream vs/differences StreamWriter?

A FileStream is explicitly intended for working files.

A StreamWriter can be used to stream to any type of Stream - network sockets, files, etc.

ScottGu explains the different Stream objects quite nicely here: http://www.codeguru.com/Csharp/Csharp/cs_data/streaming/article.php/c4223

They are two different levels used in outputting information to known data sources.

A FileStream is a type of Stream, which is conceptually a mechanism that points to some location and can handle incoming and/or outgoing data to and from that location. Streams exist for reading/writing to files, network connections, memory, pipes, the console, debug and trace listeners, and a few other types of data sources. Specifically, a FileStream exists to perform reads and writes to the file system. Most streams are pretty low-level in their usage, and deal with data as bytes.

A StreamWriter is a wrapper for a Stream that simplifies using that stream to output plain text. It exposes methods that take strings instead of bytes, and performs the necessary conversions to and from byte arrays. There are other Writers; the other main one you'd use is the XmlTextWriter, which facilitates writing data in XML format. There are also Reader counterparts to the Writers that similarly wrap a Stream and facilitate getting the data back out.

What is different between FileStream and StreamWriter in dotnet?

A FileStream is a Stream. Like all Streams it only deals with byte[] data.

A StreamWriter : TextWriter is a Stream-decorator. A TextWriter encodes the primitive type like string, int and char to byte[] and then writes hat to the linked Stream.

What context are you supposed to use it? What is their advantage and disadvantage?

You use a bare FileStream when you have byte[] data. You add a StreamWriter when you want to write text. Use a Formatter or a Serializer to write more complex data.

Is it possible to combine these two into one?

Yes. You always need a Stream to create a StreamWriter. The helper method System.IO.File.CreateText("path") will create them in combination and then you only have to Dispose() the outer writer.

FileStream writes bytes, StreamWriter writes text. That's all.