Create an array of posts that meet criteria of custom fields

Well i see a lot of posts that come close to what i need but i can’t figure it out. What i want to do is this. I want (within a loop but without disturbing it) to select a new set of posts based on a custom field. So, for instance, on my index i want to make a sub-set of posts that have a custom field called “foo”. Is wp_query what i am looking for? If so can you give me an example? If not can you point me to the right direction?

Thanks in advance guys…