English language learners are, in fact, a very diverse group in many different ways. These students enter school with a wide variety of language proficiency in both English and their native language. They also are diverse in their academic backgrounds. For instance, some students are literate in their native language and may have already had extensive instruction in certain content areas such as math and science, where they meet or exceed the standards for their equivalent grade level. Other ELLs, however, may have never attended become proficient in their native language or received any formal schooling due to living in a war torn nation or refugee camps before fleeing to a safer place. Another difference among these learners is in their time in the United States. Some students may be new arrivals to the US, while others may have lived here for several years or even been born here. According to the book Making Content Comprehensible for English language Learners: The SIOP Model by Echevarria, Vogt, and Short, these learners may also be diverse in their "expectations of schooling, socioeconomic status, are of arrival, personal experiences while coming to and living in the United States, and parents' education level and proficiency in English" (pg. 4, 2013.) All of these factors result in English language Learners being a very heterogeneous group of students with many differences as well as unique characteristics.
References:
Echevarria, J., Vogt, M., & Short, D. (2013). Introducing the SIOP Model. In Making Content Comprehensible for English Learners: The SIOP® Model (4th ed., pp. 4). Boston, Massachusetts: Pearson.
Parsons, S. (2012). ESL/ESOL Students. Retrieved October 11, 2015, from http://sp5446educ2120.weebly.com/eslesol-students.html