Banner advertising is simply the process of purchasing banner space on other websites for you to display a banner about the product or service you wish to advertise.
There are millions of websites to choose from when placing a banner advertisement. You should ultimately consider the website’s: traffic, page views, impressions, demographics, the CPM (cost per thousand impressions), monthly fee or CPA (cost per action).
The easiest comparison to a banner ad on a website would be a billboard. When considering a billboard, would you choose the busiest highway where your billboard will receive plenty of unique viewers or a side street? The more traffic on a highway would drive more unique users seeing your billboard, which is ideally what you want. You would also pay more for that premium ad space.
You can either do the research yourself, by identifying the right websites to make banner ad placements on, or you can advertise through an ad network which have a network of websites who will publish your banner ad on their sites. Utilizing a network can be more cost effective and it allows you to have a larger audience because of the number of websites on which these ad networks place you.