A "webinar" is a WEB
, i.e. a seminar conducted over the Internet. It allows participants to attend from their office or home or anywhere they have a computer and internet access. What makes webinars so popular is that participants can attend a seminar without the travel expense.
A "webcast" by contrast lacks the interactive feature. A webcast delivers a lecture over the internet but viewers cannot ask questions. In webinars, attendees can participate by emailing questions to the host which can then be answered during the course of the webinar.
For those who wonder what the three "w" at the beginning of a website address (such as www.davis-stirling.com) stands for, www = world wide web.