Difference between passing an object as parameter to a webservice method and passing the object as a SoapHeader to the method..

Question

What is the difference if we pass an object in a method, in ASP.NET WebService, and passing the object as a SoapHeader?

For details, ex:

Passing as a method Parameter:

[WebMethod]
public void Createactivity(User objUser)

Passing as a Soap Header:

[WebMethod(Description = "Sends the SMS to Mobile")]
[SoapHeader("objSMS")]
public string SendSMS()
{