There really isn't much difference between the two sentences, they are both valid. They both mean the same thing and you will get the same answer for both of the questions.

Did you eat something this evening? Did you eat anything this evening? They are similar in meaning , but the reason behind these questions can be different in certain contexts "Did you eat anything" is a "stronger" question with the choice of the word "any", which enquire further if you have eaten any thing or not. Say , If you know all the food is dirty , you would ask your friend " Did you eat ANYthing " because you are actually asking "did you eat ANYthing or NOT.> this a more urgent question than using "something". Did you eat something this evening?> its a casual question to know if she has eaten (something)