What does Streams do when implementing AES in .NET?

The Rijndael encryption algorithm is implemented in .NET using three streams in the following example: Rinjdael .

Can someone explain to me what these threads are doing? How / Why are they used?

// Declare the streams used
// to encrypt to an in memory
// array of bytes.
MemoryStream msEncrypt = null;
CryptoStream csEncrypt = null;
StreamWriter swEncrypt = null;

// Declare the RijndaelManaged object
// used to encrypt the data.
RijndaelManaged aesAlg = null;

try
{
    // Create a RijndaelManaged object
    // with the specified key and IV.
    aesAlg = new RijndaelManaged();
    aesAlg.Key = Key;
    aesAlg.IV = IV;


    // Create a encryptor to perform the stream transform.
    ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

    // Create the streams used for encryption.
    msEncrypt = new MemoryStream();
    csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write);
    swEncrypt = new StreamWriter(csEncrypt);

    //Write all data to the stream.
    swEncrypt.Write(plainText);

}