Banner ads have been around since the beginning of the internet. Some sites are covered in them. Giant companies like Google or Amazon offer a incentive to display their ads on your site for a small fee based on how many clicks your site generates and other factors.
If you would like to offer clients banner space for them to advertise on your site, a great way would be the AdRotate plugin for WordPress. The AdRotate plugin, make sit easy for you to start your own banner campaigns. You can sell any size banner ad to your clients.
It is simple to set, and has extensive tracking options as well, for instance you can set how long the banner swill run or the number of impressions per page. Along with this you can track clicks and make groups, so that certain groups only appear in certain sections of your site.
There is a little bit of a learning curve, and some knowledge of HTML can help you make it look even better. But when wanting to start your own banner campaigns for clients, AdRotate plugin for WordPress is a great option.