It's Greasemonkey for native apps, or, put in more technical terms, it's a dynamic code instrumentation toolkit. It lets you inject snippets of JavaScript into native apps on Windows, Mac, Linux and iOS. Frida also provides you with some simple tools built on top of the Frida API. These can be used as-is, tweaked to your needs, or serve as examples of how to use the API.

Great question. We'll try to clarify with some use-cases:

• There's this new hot app everybody's so excited about, but it's only available for iOS and you'd love to interop with it. You realize it's relying on encrypted network protocols and tools like Wireshark just won't cut it. You pick up Frida and use it for API tracing.
• You're building a desktop app which has been deployed at a customer's site. There's a problem but the built-in logging code just isn't enough. You need to send your customer a custom build with lots of expensive logging code. Then you realize you could just use Frida and build an application- specific tool that will add all the diagnostics you need, and in just a few lines of Python. No need to send the customer a new custom build - you just send the tool which will work on many versions of your app.
• You'd like to build a Wireshark on steroids with support for sniffing encrypted protocols. It could even manipulate function calls to fake network conditions that would otherwise require you to set up a test lab.
• Your in-house app could use some black-box tests without polluting your production code with logic only required for exotic testing.

Frida's core is written in C and injects Google's V8 engine into the target processes, where your JS gets executed with full access to memory, hooking functions and even calling native functions inside the process. There's a bi-directional communication channel that is used to talk between your app (Python?) and the JS running inside the target process.

On top of this C core there are multiple language bindings (Python, .NET and a browser plugin), and it is very easy to build further bindings for other languages and environments (Node.js could be a future binding if anyone's interested in helping out with that).

Have a look at our Quick-start Guide.