I often hear certain conlangs (e.g. Esperanto) referred to as auxlangs. What exactly is an auxlang? How can I tell if a conlang is an auxlang? What are some examples of auxlangs?
Auxlang is a short word for "auxiliary language", i.e., a language that is designed and promoted as a bridge language between people of different languages.
Examples for international auxiliary languages are Volapük, Esperanto, or Interlingua; examples for zonal languages are Interslavic or Afrihili.
According to Wikipedia, auxlangs are languages intended to facilitate international communication. Some notable examples include Esperanto and Interlingua. Of these, Esperanto is the most well-known and successful. Zamenhof created it in the 1880s.
Edit: As pointed out in the comments by @RadovanGarabik, these are only some notable examples of constructed auxlangs. See the comments for examples of natural ones.