Hot-Standby: Ability to query against what used to be Warm-Standby boxes which were for disaster recovery only. Streaming Replication: sending log data over sockets instead of relying on shell commands to send logfiles over.

They work together to make for happy master-slave replication.

It's worth noting this is database server level. Hot-standby copies 100% of a PostgreSQL instance. If you want to replicate individual databases on a server or individual tables in a database, Longdiste, Bucardo or Slony are still the way to go.