A webinar is a word that mixes the 'web' with the word 'seminar'. Getting, you guessed it – 'webinar'. So it's an 'online seminar'. So instead of going to a hotel business conference centre in the city centre over lunch; you:
Sign up / register to attend the webinar;
Get an email with a link for you to click to join the webinar at the chosen time;
And then you join it!
Good webinars allow you to ask live questions through the webinar; and to see the screen being shared by the person presenting the webinar. Good webinar practice means that you should then be able to see the webinar again in a recorded version over the web.
Interesting fact: the word 'seminar' is derived from the Latin word seminarium, meaning "seed plot"; and by 'seeds' they mean the ones like sunflower seeds that you plant in the ground, and see a beautiful sunflower appear a few months later; and by 'plot' it means a little 'plot' or area of land. So you could say that a webinar is just a little plot of land on the internet, where seeds are sown, and from which great flowers blossom.
Just saying J