The :event key is the important bit here. The keys prefixed with colons are just the internal logstash logging decoration.

{
  :timestamp=>"2014-10-20T17:52:29.931000+0000",
  :message=>"output received", 
  :event=>{
    "host"=>"deployment-parsoid04",
    "short_message"=>"started parsing",
    "facility"=>"parsoid",
    "level"=>6,
    "full_message"=>"{\n  \"name\": \"parsoid\",\n  \"hostname\": \"deployment-parsoid04\",\n  \"pid\": 26612,\n  \"level\": 30,\n  \"type\": \"info\",\n  \"location\": \"[info][enwiki/Black_cat?oldid=52615]\",\n  \"msg\": \"started parsing\",\n  \"time\": \"2014-10-20T17:52:24.900Z\",\n  \"v\": 0\n}", 
    "version"=>"1.0",
    "@version"=>"1",
    "@timestamp"=>"2014-10-20T17:52:24.900Z",
    "source_host"=>"10.68.16.17",
    "pid"=>26612,
    "type"=>"info",
    "location"=>"[info][enwiki/Black_cat?oldid=52615]"
  },
  :level=>:debug,
  :file=>"(eval)",
  :line=>"1199"
}