Generator expressions in Python are a concise way to create generator objects, which are iterators that yield values one at a time. They are similar to list comprehensions but use parentheses instead of square brackets. This allows for the generation of values on-the-fly, making them memory-efficient, especially when dealing with large datasets. Unlike lists, generator expressions do not store their contents in memory, which can significantly reduce memory usage. They are particularly useful when you need to process data sequentially without loading everything into memory at once, making them ideal for handling large streams of data or complex computations.