I got my server being hacked because I leave elasticsearch listen openned for the Internet.
index: an index is like a database in relational database.
type: a type in like a table in relational database.
document: document is a JSON document which is stored in elasticsearch.
node: a node is a running instance of elasticsearch which belongs to a cluster.
cluster: a cluster consists of one or more nodes which share the same cluster name.
shard: a shard is a single Lucene instance. it is a low-level "worker" unit which is managed automatically by elasticsearch.