More and more innovative types of advertising are coming into existence as the Web matures, so this is not meant to be an exhaustive list. However, it covers all the main types of advertising in use today.
Despite widespread concern about the effectiveness of banner advertising, this is still a popular form of web advertising, perhaps because it's so familiar.
A banner ad is a graphical bar or button containing text or graphics designed to attract a viewer's attention and induce an action (usually, invite the viewer to click through the banner and visit the advertiser's site).
Banners come in all shapes and sizes, although some clear standards have emerged. For instance, the 468x60 pixel banner is a very popular size, as is the 234x60 banner (half the size of the larger banner, so two fit into the same space). Here's a list of banner sizes to help you.
Banner ads can be static or animated. The most popular graphic format for banner ads is GIF format. Most banner ads are sold on a CPM basis or on a CPC basis.
Some sites show more than one banner ad per page, and some even go so far as to use a little software program to rotate banners every few seconds while a visitor remains on a given page.
Probably the most well-known example of text ads is Google's Adsense program, which delivers contextually relevant text ads (ads targeted based on the content of a page) and is in use by hundreds of thousands of sites worldwide.
Text ads in newsletters are usually specified as a number of lines, with a maximum number of characters per line. For instance, a site might offer ad space in its newsletter by specifying a maximum of ten lines of text and 65 characters per line.
You'd think that marketing types had nothing better to do than dream up swanky terms. Interstitial ads are nothing more than ads that are shown in the transition between two pages of a site. So you click on a link on Page A, but instead of going to Page B you arrive at an intermediate page containing the sales pitch (and - hopefully - a link to Page B somewhere on the page!). Interstitials are gaining in popularity with advertisers since they offer an almost unlimited amount of space to pitch a product. Many site visitors find interstitials irritating, and they also increase site loading times.
According to many webmasters, pop-up ads are the most annoying type of advertisement... although there is little evidence this sentiment is shared by the larger Web community.
Pop-up ads consist of a small window that "pops up" over the main browser window when you enter a site (and sometimes when you leave it, a favourite tactic of adult sites). The pop-up windows can contain anything: text, graphics, a form to collect information or email addresses, even a little game.
There are two downsides to pop-up ads, one for webmasters, one for advertisers. From a webmaster's point of view, pop-up ads wrench control of the browser away from their own page, and some badly-written pop-up ads may also crash certain browsers, leaving a permanently bad impression in a visitor's mind. From an advertiser's point of view, most pop-up windows can be minimized (hidden behind the other windows) with relative ease, so if the pop-up window is being used to rotate ads on a time basis, your advertisement may not even be visible but you'll still be charged for it!
An advertising type that is rapidly becoming popular, opt-in mailing consists of sending an email message to a "pre-qualified" list of people i.e. an audience that has expressed an interest in receiving information on a given topic.
Some sites sell their lists of newsletter recipients to advertisers, but most prefer to keep the email addresses secret and distribute the ad on the advertiser's behalf.