Great questions! Please allow me to answer them each one by one:
What is Linchpin?
Linchpin is a feature rich social Intranet solution that uses Confluence as its foundation. You may want to check out the information on what Linchpin is.
Is Linchpin a set of Confluence plugins?
As an intranet solution, Linchpin is comprised of up to 15 modular Confluence plugins. Each plugin has it's own unique feature set tailored to specific use cases. Used as a whole, or in key subsets, these plugins are being used by many of the worlds leading companies to solve intranet and corporate communications challenges.
How does Confluence differ from Linchpin?
Confluence is the foundation for Linchpin. That said, there are some key differences. Linchpin retains the core functionality of Confluence (collaborative and easy to use rich text editing, knowledge management, etc...) and adds to it things like personalized news, personalized dynamic navigation, complete corporate branding options, a personalized app store, and more. There is a nice table posted here that helps show how much more Linchpin adds to Confluence and Linchpin Comparison. There is also a bigger comparison table that deciphers between Confluence, Linchpin and Microsoft SharePoint.
Can I just install some Atlassian Marketplace apps and get Linchpin as a result? If so, which are these?
In essence, yes. You can buy the core Linchpin apps directly from the marketplace and install them. In fact, the process to do so is simply outlined in a step by step guide here: Generating your Linchpin Intranet Instance from Scratch.
The "core Linchpin apps" are as follows:
Check out the pricing details linked from this price overview. The details contain all apps and links to the Linchpin apps on the marketplace by the vendor.