Explain multithreading vs multiprocessing

In the context of implementing a crawler, answer the following:

1. What is the difference between multithreading and multiprocessing ?
2. How do these relate to running on a multi-core CPU ?
3. For a network crawler, when would you prefer threads vs processes?
4. What are common pitfalls (e.g., race conditions, deadlocks, overhead), and how do you mitigate them?