From my point of view, these two (TESL and TEFL) are the same. When we teach, we'll definitely employ different strategies compared to teaching English to the native speakers. Most of the time, the approaches that a teacher employs are much depending on their learners as well. For example, when you teach English to speakers of other languages, the purposes of the students learning that targeted language must be taken into consideration. If the students are merely learning for communication purposes, then the design of the lesson will be focusing on interaction and more on communicative activities.
Back to your question, at times it's rather hard to determine whether English is the second or foreign language to a learner as well. Due to education or government policy, English could also be taught as a second language when the fact is it should be a learner's foreign language. For instance, Malaysia. English is a foreign language for many children in Malaysia. Let me quote myself as an example. Mandarin is my mother tongue and my second language is Hokkien. Malay language is my third language and obviously English is a foreign language to me when I learnt it in school. However, English is the second major language used in the goverment sector and also by most of Malaysians. Consequently, English is not considered as a foreign language and it's taught as a second language under Malaysia Education System.
As a teacher, correct me if I'm wrong, we basically use the same approaches when teaching English to these two group of students.
