Caribou is not a single library, but an ecosystem of interacting components, each of which has the ability to stand on its own. This idea lends a quality of composability to the Caribou world. If some capability does not exist, it can be created on its own and then easily linked into a working Caribou instance.
That said, there are some core components that lay the foundation for everything that follows.
Caribou Core lays the foundation for all of the other libraries by capturing the data model of a site as data! A data model is traditionally only an implicitly defined being, existing as a conglomeration of migrations and tables whose relationships are only formed through happy accidents within application code. Caribou Core structures the data model in a way that allows it to serve a variety of other purposes, including the construction of queries that filter and order based on the relationships between models.
Caribou Core can be used on its own if all you need is the dynamic models as an interface to a database backend. In practice it is usually supporting a site running the Caribou Frontend, Admin and API, but nothing is stopping you from using it independently of an HTTP Ring server.
The heart of Core is the Model system, which abstracts over a database schema and provides methods for making schema transformations through transformations on pure clojure data. In every way, Model (which represents a database table) itself is a Model, with an association to Fields (which represent the columns of a database table) which is also a Model. This is a radical choice, and lays the foundation for the rest of Caribou's power. The ability to treat Models and Fields themselves as data enables Caribou to generate an Admin and API for you automatically, and countless other benefits that you will discover as you go deeper into the Caribou ecosystem.
Caribou Frontend uses Core as the data layer foundation and is built on the Clojure Ring protocol. Ring is a flexible HTTP protocol for Clojure that abstracts over the HTTP request and response lifecycle, turning them into plain Clojure maps. In practice this is an extremely powerful way to compose handlers and functionality into a robust web server.
Frontend adds onto the Core and Ring base a fully-functional routing, controller and rendering system. If Core is the M, then Frontend is the VC. At the heart of this system is the Page, which associates routes to the controllers that run when they are matched and the templates that are ultimately rendered with data retrieved and defined in the controllers.
Caribou Admin provides a browser-based interface to all of the Caribou functionality. Things you would previously need someone to code for you can be done with the click of a button. Adding new Models, adding new Fields to those Models, creating content based on those Models, adding Pages for routing and rendering, localizing content for many languages and locales, adding Accounts and managing Permissions, all of this is accessible through the Admin interface. No need to build a custom admin for every project! This alone cuts down on the development time of a project by a large degree, and is one of the huge advantages of using Caribou to build your site.
Caribou API provides a RESTful API in a variety of formats (json, xml or csv) which tap into any content you create in Caribou. Create a new Model and instantly an endpoint representing that Model is available. Add some content for that Model, the content magically appears in the API results. Use any of the options for filtering and selecting content as URL parameters that would previously only be available programmatically. The API again is a tangible upshot of Caribou's Model-as-Data approach.
Besides the base libraries, there is a whole tundra of associated libraries that exist on their own, but also contribute to the Caribou ecosystem.
Lichen is a standalone image resizing library that enables Caribou to define image sizes during template creation. Lichen creates the newly resized version of the image the first time it is requested, then reuses the cached version on each subsequent request transparently, so the developer never needs to worry about it. Declare what you want, Lichen worries about how to most efficiently perform the task.
Antlers is a templating library that grew out of the raw Mustache spec, but adapted to ever-expanding demands from day-to-day use. Today it is a fully functional templating system with blocks, helper functions, loop variables and a host of other practical features that make it the cornerstone of rendering in Caribou. Of course, nothing is stopping you from using whatever template system you want, but if you need it, it's there.