What is the difference between generator expressions and list comprehensions in Python?

Generator expressions and list comprehensions are both methods used to quickly generate lists, but they have several important differences.

1. List comprehensions immediately create a complete list, while generator expressions are lazy generators that only generate values when needed. This means that generator expressions are more memory efficient, especially for large datasets.
2. Generator expressions use parentheses, while list comprehensions use square brackets.
3. Generator expressions can only be used in generator functions and other functions that take iterable objects (such as sum(), map(), filter(), etc.), while list comprehensions can be used anywhere a list is needed.

In conclusion, using a list comprehension is suitable for immediately creating and iterating over a complete list, while a generator expression is preferable for delaying value generation and using memory more efficiently.