What difference between NEventStore and EventStoreDB

Additionally to mentioned EventStoreDB and NEventStore there is relatively new SQLStreamStore library, that conceptually similar to NEventStore.

They have a few essential differences, but historically SQLStreamStore can be consider as a successor of NEventStore. The author of the SQLStreamStore damianh previously was a maintainer of NEventStore. The latest release of NEventStore was 15 Mar 2014, SQLStreamStore is actively developing.

For more details why SqlStreamStore was created see the answer SQLStreamStore vs NEventStore vs EventStore #108

NEventStore is a persistence agnostic event sourcing library for .NET with multiple storage options such as relational and document databases.

EventStoreDB is a newer and high performing event sourcing solution with its own persistence engine and API's for multiple protocols and languages. It has open source and commercial options.

As far as I know you can use EventStoreDB as a persistence engine for NEventStore in a future version of NEventStore as well.