There are many different terms and abbreviations used in discussing the topic of second language acquisition. Just some of these include second language learning, L2 acquisition, ELL (English language learners), and ESL (English as a Second Language). ESL and ELL are sometimes used interchangeably, and sometimes argued to be completely different things. ESL seems to be an older term that, depending upon the source, is either being phased out, or is continuing to be used to distinguish a specific pull-out program, as opposed to somebody in the general education environment who happens to not speak English. Some claim that ELL is more politically and technically correct, since English could be a third or fourth language. In all my years I’ve never experienced any language issues with a student learning English as a third or fourth language (I mean actual language disorders where the language deficits occur in similar dimensions across all languages), but I suppose it is technically possible. Also, use of these terms seems to be different in different places. There is a good little description of ESL and ELL issues in this link.
One of the preeminent researchers in second language acquisition is Stephen Krashen. According to Krashen, learning is less important than acquisition. His theory includes five main hypotheses, which he’s labeled the Acquisition-Learning hypothesis, the Monitor hypothesis, the Natural Order hypothesis, the Input hypothesis, and the Affective Filter hypothesis. His Affective Filter hypothesis embodies one of his main views that a number of affective variables play a facilitative, but non- causal, role in second language acquisition. These variables include: motivation, self – confidence, and anxiety. You can find a lot of his stuff at his site.
Second language acquisition presents some interesting challenges for those who teach language. In school settings, speech-language pathologists are supposed to only work with students with disabilities. For students whose primary language then is something other than English, this means that a language disability should exist in that student’s first language in order to qualify for services. Theoretically and legally, the disability should have nothing to do with the fact that the student has learned another language prior to English. In the real world, it gets complicated. Some kids do all right with their first language in preschool, and then face problems as parents may attempt to use more English at home. Maybe one parent speaks more English. Aunts, uncles, grandparents, friends, etc. all bring their own language preferences and abilities to the mix. Then there are things like code-switching, the switching between languages in a conversation or with different conversation partners. Commonly, these kids also display a silent period, in which they are so focused on comprehension that they don’t speak much. Also, there can be language loss of the first language if it is not continuously reinforced. There have been controversies over the extent to which academics should be taught in one language over the other, as well as the extent to which English must be learned, and who is responsible. I think most experts agree that bilingualism is an awesome attribute. More info can be had here.
As an interesting aside, this recent study suggested that second language learners may have an advantage in learning to read compared to native language speakers. The study’s authors suggested that this may be due to an increased awareness in language overall – metalinguistic awareness.