Sample Code:

{!REQUIRESCRIPT("/soap/ajax/32.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/32.0/apex.js")}

var mail = new sforce.SingleEmailMessage();

mail.targetObjectId = "{!Contact.Id}";
mail.toAddresses = "{!Contact.Email}";
mail.templateId = "00X28000000Utjy";
mail.whatId = "{!Case.Id}";

var result = sforce.connection.sendEmail([mail]);

if (result[0].success == 'true') {
    alert("Email sent successfully.");
} else {
    alert("Email sending failed.");
}

Note: Only User, Contact, Lead, or Person Account objects are allowed for targetObjectId.

If you need to refer specific attributes then download Partner WSDL and refer complexType SingleEmailMessage.

<?xml version="1.0" encoding="UTF-8"?>
<complexType name="SingleEmailMessage">
   <complexContent>
      <extension base="tns:Email">
         <sequence>
            <element name="bccAddresses" minOccurs="0" maxOccurs="25" type="xsd:string" nillable="true" />
            <element name="ccAddresses" minOccurs="0" maxOccurs="25" type="xsd:string" nillable="true" />
            <element name="charset" type="xsd:string" nillable="true" />
            <element name="documentAttachments" minOccurs="0" maxOccurs="unbounded" type="tns:ID" />
            <element name="entityAttachments" minOccurs="0" maxOccurs="unbounded" type="tns:ID" />
            <element name="fileAttachments" minOccurs="0" maxOccurs="unbounded" type="tns:EmailFileAttachment" />
            <element name="htmlBody" type="xsd:string" nillable="true" />
            <element name="inReplyTo" minOccurs="0" type="xsd:string" nillable="true" />
            <element name="optOutPolicy" type="tns:SendEmailOptOutPolicy" nillable="true" />
            <element name="orgWideEmailAddressId" minOccurs="0" maxOccurs="1" type="tns:ID" nillable="true" />
            <element name="plainTextBody" type="xsd:string" nillable="true" />
            <element name="references" minOccurs="0" type="xsd:string" nillable="true" />
            <element name="targetObjectId" type="tns:ID" nillable="true" />
            <element name="templateId" type="tns:ID" nillable="true" />
            <element name="templateName" minOccurs="0" type="xsd:string" nillable="true" />
            <element name="toAddresses" minOccurs="0" maxOccurs="100" type="xsd:string" nillable="true" />
            <element name="treatBodiesAsTemplate" type="xsd:boolean" nillable="true" />
            <element name="treatTargetObjectAsRecipient" type="xsd:boolean" nillable="true" />
            <element name="whatId" type="tns:ID" nillable="true" />
         </sequence>
      </extension>
   </complexContent>
</complexType>