What is neptune?

Neptune is structured data storage. It's DBMS(Database Management System), but not Relational DBMS.

How Neptune is different from Hadoop Fils System.

File system like Hadoop can store flat file. It is impossible to retrieve a specified record within milliseconds in a flat file.
"Structured" means indexed. RDBMS like Oracle is most popular structured data storage.
You can store structured(indexed) data in the Neptune and can retrieve within a few milliseconds.

How fast online operation?

Neptune is not optimized for online operation. But supports resonable speed.
On average workload status, You can put a row(1000 bytes) within 0.5 ~ 5 ms and can get within 1 ~ 5 ms.
Look performance test results.