What are .NSF files?

NSF stands for "Notes Storage Facility". The system architecture is roughly as follows:

+----------------+
| Notes (client) |
+----------------+
         |
+----------------+
| Domino (server)|
+----------------+
         |
+----------------+
|   .NSF file    |
|(Notes database)|
+----------------+

So an NSF file is just the database that Domino uses. Even the term "database" is a misnomer. The NSF contains security rules, application code and data all in one file, and can also normally be accessed directly from the client without using a Domino server. Each Domino application/website/whatever can be one or more databases.
NSF/Notes/Domino is not a scripting language and is not similar to ASP/PHP. The Notes/Domino system is fairly unique. It's an entire infrastructure that handles security, storage, scripting etc.
In terms of development languages, you can use C, Java, LotusScript (kind of like VBScript), Formula Language (a proprietary very high level language) and one or two others.

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