In some respects they are the same, but Eloquent is ORM that uses the Query Builder, while Query Builder is fluent interface for building queries. If your desired result is a Model and you want to use the behaviours on the Model, then Eloquent is needed, Whereas Query Builder will give you basic stdClass instances with properties, and no Model behaviours.
Broadly, I would say that Eloquent is an object representation of a (collection of) record(s) in a table, a User or _ a Post_ etc. Your result will be an instance of the given Eloquent model and will have the ability to load nested relations, accessors and mutators etc.
Whenever the result you want is more than that, e.g. aggregations, multi-table joins etc. it will more often use Query Builder where you translate a SQL query into its Query Builder representation. You will have a flat data structure, with raw data from the query.