I think that JN was not speaking literally when he asked what is " a ESL;" that is, it was "tongue in cheek" because there isn't such as a thing as "a ESL." Albeit, since you have addressed the issue of whether or not there is a difference, I can tell you, based on my experience as an ESL teacher in America and an EFL teacher in China, there is an extreme difference in approaches.

When I first came to China, although I had training in TESOL and TESOFL, I still felt I could rely on ESL methods that I had used successfully in America. To some degree, yes; however, overall, I had to rethink that idea and after five years of teaching EFL in China I'm still rethinking and adapting.

Of course the most obvious difference is that ESL is taught to those who by the necessity of having emigrated to an English speaking country will need to learn English in order to function properly outside of their own language community. Since they have the opportunity to be immersed in an English speaking environment, the task of the ESL teacher, especially at the lower levels can be more focused on the specific needs in terms of enabling students to function. Moreover, and this is true at all levels, extension activities can be assigned that will enhance the classroom activities. On the other hand, as we are all aware, it is quite difficult to get Chinese EFL learners to use anything but Chinese outside of the classroom. Therefore, our lesson planning has to be geared to maximum effect within the classroom environment.

In the interest of brevity, I'll leave it at that. However, I am always interested in hearing what others have to say about this subject and how they have had to adapt to the differences between ESL and EFL.