Webinar refers to web-based seminar and is a relatively new concept. Webinar allows for a presentation, lecture or workshop to be conducted online. Given the busy lifestyles of working professionals and students, web-based content dissemination has a broad appeal. Webinars also remove geographical barriers and can reach anyone anywhere in the world. Another advantage of the webinar is the accessibility of the content at on consumer's desired schedule. As long as the information is available on the webinar link, any interested person (who has been allowed access) can download it or view it online.
Industry and academia are the major users of webinars. The biggest application of webinar is for MOOCs (Massive Open Online Courses), courses that are available for mass-scale participation and open access via web. Coursera and EdX are the main platforms for MOOCs and many of their MOOCs have thousands of participants (I have successfully completed 8 MOOCs in the past year). These freely available courses provide the course materials as series of short videos or webinar format that can be accessed anytime by the participants. In fact, the course completion certificates of MOOCs are honored by prospective employees and MOOCs are counted as actual coursework.
A number of softwares are available for making and broadcasting the webinars, including Google Hangout, Skype, Adobe Connect, etc.