I'm trying to decide what tags I should be using and realized I did not know the difference between these terms.
Simply put, an expression is more like a saying, an idiomatic way of expressing something in less literal, less familiar terms:
How do you do? is just an expression: you don't expect someone to actually tell you how he is doing; he should just say how do you do? back, because it means little more than "hello".
A phrase is just any sequence of words, usually less than a sentence:
- rip them out
- even the Aztec women
- before the Spaniards
- reloading muskets
- because they could never have continued their rituals without capturing large numbers of enemy soldiers (this one is dubious, because "clause" would be a more specific and thus better name)
A great many expressions are also phrases, like put behind bars or heaven forbid.