This document discusses the JSON feature object as returned by the REST API.
A feature consists of 2 properties. All properties are optional:
geometry: 
The feature geometry. 
It can be any of the supported geometry types. 
It is a JSON object as defined in the Geometry Objects doc.
"geometry" : {"x" : -118.15, "y" : 33.80}
attributes: 
The feature attributes. 
It is a JSON object that contains a dictionary of name-value pairs. 
The names are the feature field names. 
The values are the field values and they can be any of the standard JSON types - string, number and boolean. 
Note that date values are encoded as numbers. 
The number represents the number of milliseconds since epoch (January 1, 1970) in UTC.
"attributes" : {
  "OWNER" : "Joe Smith", //string
  "VALUE" : 94820.37, //number
  "APPROVED" : true, //boolean
  "LASTUPDATE" : 1227663551096 //date
}
{
  "geometry" : <geometry>,
  "attributes" : {
    "name1" : <value1>,
    "name2" : <value2>,
  }
}
{
  "geometry" : {"x" : -118.15, "y" : 33.80},
  "attributes" : {
    "OWNER" : "Joe Smith",
    "VALUE" : 94820.37,
    "APPROVED" : true,
    "LASTUPDATE" : 1227663551096
  }
}