What is the difference between IQueryable and IEnumerable in .NET Core?

In .NET Core, IQueryable and IEnumerable are both interfaces that represent a collection of objects. However, there are some key differences between the two:

1. Deferred execution: IQueryable supports deferred execution, meaning that it does not execute the query until the results are enumerated. This allows for more efficient queries, as it only retrieves the data that is necessary. IEnumerable, on the other hand, does not support deferred execution. 

2. Expression tree: IQueryable uses expression trees to represent the query, while IEnumerable uses delegates. Expression trees are more powerful and allow for more complex queries to be constructed and analyzed at runtime.

3. Query provider: IQueryable has a query provider, which is responsible for translating the expression tree into a SQL query or some other data source query. This allows for more flexibility in the types of data sources that can be queried. IEnumerable does not have a query provider and is limited to in-memory collections.

Overall, IQueryable is more powerful and flexible than IEnumerable, but also more complex to use. It is generally recommended to use IQueryable for querying external data sources, and IEnumerable for in-memory collections.