There are many different methods of online advertising. These include older more traditional methods of advertising that include banner advertising or graphic display advertising. This type of advertising can be contextual related content advertising. The difference between the two are basically this... Contextual advertising models essentially know and remember the sites and types of things that you have visited in the past and they keep showing ads to you that relate to what you have seen in the recent past. Related banner ads show you banners that are related to the content that you are presently viewing on that exact site at that time.
Contextual ads are basically what Google Adwords and Google Adsense use to display their ads. This may be because so many publishers don't really always want to show direct competitive ads on their sites. By showing the visitor some other content or brand that they have visited recently helps to build brand awareness for that company by keeping it in their frame of reference to build their sales and brand awareness.
There are many other forms of online advertising. These include things like: email advertising, social advertising, PPC or pay per click, interstitial advertising, link building, video promotion, blog marketing and advertising and more. There are lots of choices to be aware of when developing an online advertising strategy. Select a professional advertising agency that has the breadth and scope of the online advertising that will serve your business needs the most.