@XMLRootElement versus @XmlType

The difference between XmlRootElement and XmlType is a matter of scoping. Remember this annotation is merely dictating the creation of the schema used to generate your XML. The XmlRootElement denotes a global element (with an anonymous or schema type):

<xs:element name=foo type="bar"> </xs:element> <-- schema type

while the XmlType is used to denote a local element (with an anonymous or complex type):

<xs:complexType name=bar> </xs:complexType> <-- complex type

The main differences in local/global here are in the hierarchy of the schema your object will appear in and whether you are declaring a schema type or complex type. The documentation for both of these annotations is well written and includes examples:

XmlRootElement

XmlType

EDIT: Addressing the propOrder question: you can use it on a global element if you are also declaring a local type:

@XmlRootElement (name="PersonElement")
@XmlType (propOrder={"firstname", "lastname"})
public class People{
    @XmlElement
    public String firstname;
    public String lastname;
}

This will yield something like:

<xs:element name="PersonElement" type="People"/>
<xs:complexType name="People">
    <xs:sequence>
        <xs:element name="firstname" type="xs:string"/>
        <xs:element name="lastname" type="xs:string"/>
    </xs:sequence>
</xs:complexType>