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"
}