Web Based advertising can come in many forms but one of the most popular is simply to have ads on you own website for other businesses or products. These ads have been around for many years and are so widespread that people have begun to have ad blindness (not noticing ads on page because they are so ubiquitous and instead filtering them out mentally to search for what they came to find), or they have adblocker which stops ads from being displayed in the first place.
For the last several years Adwords has come to dominate the online advertising sector. When people need to find something, what do they do? They use a search engine (mostly likely google) to find it. Placing ads here, before someone reaches your website is a great way to drive new traffic to your site, because you have consumers who are already searching for what you are selling, so they will be more interested in your ads. However this is not a way to make money as a webmaster.
Large traffic websites, like youtube (owned by google), yahoo, facebook, dating sites like PlentyOfFish, large gaming sites, or some news media outlets are having success running in house ad networks, where you can buy impressions or clicks from them. Some like Facebook and PlentyOfFish let anyone advertise who follows their guidelines. These can be worthwhile to use, if you are doing slightly more niche advertising, that is rethinking your campaign to change it from search. The media outlet sites often place ads for a single product, who has purchased all their ad space, like a newly released movie or game. Or they have ads for companies that advertise through them in more than one format (ie print, tv, radio, etc.). They continue to be a good investment because of the numerous options they give you and their ability to deliver your message at a large scale.
Adsense has been around for many years and can be a good simple option, if you aren’t looking to make it rich. Adsense pays for clicks that you send through ads on your site to another site. Google acts as a middleman for these transactions and decides what ads go on your site. For most websites it is unfeasible to sell ads to advertisers on their domain. They simply cannot provide the traffic or conversions that would make it worthwhile. Unless you can guarantee a certain amount of traffic and track that you deliver it, getting business to advertise on smaller sites, would most likely have to be done on a PPC basis because you cannot guarantee the traffic or impressions of larger sites/networks, so the investment is no longer worthwhile.
Any business can now look online and find out the traffic to a website from places like alexa.com, and they can find the amount of search traffic it would most likely get through Google, so they can make decisions based on the data. And the data will most likely tell them to advertise with Google if they want to pay, or continue to advertise/brand themselves for free on social channels. Smaller sites cant deliver what the large sites do, or what search engines do, or what social media does, and selling ads on a local niche website is no longer a very viable option for webmasters. Chances are you are better advertising about yourself.