This ecosystem is super confusing. Here is the low-down:

• LevelDB is a (K/V-Store) database
• leveldown is a NodeJS binding.
• levelup is another NodeJS module, which wraps leveldown. However, it allows leveldown to be replaced with another backend.

In this way, you can use a single library and API – levelup – but with all kinds of different storage backends,  from Redis, to SQL databases.