ESL. How does it work, exactly?

So far, all I know about my TFA assignment is that I am teaching ESL. I still don't know whether I will be teaching elementary or secondary.

When I applied to TFA I listed elementary as my first choice and I was 99% certain that I would be assigned to that since my degree is in Italian (therefore I'm not qualified to teach, say, science or history). Now, being assigned to ESL, there is a possibility that I could teach high school. I would still be OK with that, though I had imagined myself teaching 3rd grade (because I love that age) or 4th or 6th grade (because those were the years I had my two favorite teachers).

I'm not sure what it will be like to teach ESL or even what an ESL classroom is like since I've never been in one. I vaguely remember there being an ESL class in my junior high school. (For some reason we had a large number of Russian and Ukrainian families in our area). So, I imagine that in high school or middle school I would teach a few classes to a few different groups of students, but how would it work in an elementary school? Wouldn't ESL students be integrated with the rest of the students? Would I teach a "mixed" class such as that? Or, would I maybe be a sort of "floating" teacher working with students in other classes? I'm guessing that the situation is different for each school/district. How does it work in your schools? I spent some time today looking for a blog by an ESL/ELL teacher but didn't have much luck. Anyone know of one?