A survey is an investigation into the opinions and/or experiences of a group of people (the target) done by asking them a series of standardized questions (the questionnaire). Surveys are useful for gathering research and ideas on a specific topic. Surveys may be online (through a website) or offline* (paper-based).
Here are a few reasons why teachers and other TEFL workers might need to write surveys:
What's the difference between a poll and a survey? Essentially, a poll is a "quick survey", answering only one question. A survey can answer more questions in more depth.
* Some offline surveys are conducted in person with a researcher asking the respondent questions and entering their responses. Such survey techniques generally allow for added complexity and it is beyond the scope of these pages to discuss them.