Analytics extensions cater usecase specific logics that are not out of the box available in Siddhi. For example if you want to do natural language processing that analyse or derives meaning from some English language input, you can use the nlp extension with Siddhi.
Input event adapters are used to receive events from event sources. For example if you want to use rabbitmq server as event source, you can use rabbitmq event input adapter to receive events.
Output event adapters are used to send events to event sinks. For example if you want to persist the data in HBase database, you can use HBase output event adapter to send events.
The following is a complete list of analytics extensions.