[GordyMD]

See the FAQ http://horizon.io/faq/

For convenience, this is the section taken from it that addresses this exact question! (I imagine other people may be thinking this)

How is Horizon different from Meteor?

Horizon has philosophical and technical differences with Meteor that will result in vastly different developer experiences.

Horizon is a small layer on top of RethinkDB with a narrow, purpose-built API designed to make it very easy to get started building realtime apps without backend code. Horizon isn’t prescriptive – you can use any front-end framework without any magic/customizations, and once your app outgrows the Horizon API you can use any backend technology (e.g. Node.js, Python, Ruby) and any backend framework (e.g. Rails, Express, Koa, etc.)

By contrast, Meteor is a much more prescriptive framework. Horizon is a reasonably small component that has a very clean separation with the database and the frontend, while Meteor is a much more monolithic experience.

Another major difference is architectural – Meteor uses a LiveQuery component built by tailing MongoDB’s oplog. This approach is fundamentally limited – it’s impossible to do many operations efficiently, and even the basic functionality is extremely difficult to scale.

Horizon is built on RethinkDB, so the LiveQuery functionality is in the database. This allows for much more sophisticated streaming operations, and scalability is dramatically simpler because the database has all the necessary information to allow for a scalable feeds implementation.