Is Celery mostly just a high level interface for message queues like RabbitMQ? I am trying to set up a system with multiple scheduled workers doing concurrent http requests, but I am not sure if I would need either of them. Another question I am wondering is where do you write the actual task in code for the workers to complete, if I am using Celery or RabbitMQ?
Celery is the task management framework--the API you use to schedule jobs, the code that gets those jobs started, the management tools (e.g. Flower) you use to monitor what's going on. 
RabbitMQ is one of several "backends" for Celery. It's an oversimplification to say that Celery is a high-level interface to RabbitMQ. RabbitMQ is not actually required for Celery to run and do its job properly. But, in practice, they are often paired together, and Celery is a higher-level way of accomplishing some things that you could do at a lower level with just RabbitMQ (or another queue or message delivery backend).
What is the relationship between Celery and RabbitMQ?