Well firstly, you see the ad on top, and the ad just to the left? If you don't, turn off your Adblocker (you cheap bastard). Those are Internet Ads. They are basically like ads in print media. They allow money to come to a site, so that sites can operate without people charging us to use the site (or at least charging us less than they would have).
In its most basic form, an ad on the Internet is simply put in a designated spot. Two main differences between this and print ads are that money from the ad is partly based on how many clicks an ad gets (so do that once in a while, to help this site), and that an ad will likely change if you refresh the page, especially when more companies are advertising on the site. Unfortunately, the pay-per-click model invites something called click fraud. Project Wonderful, which also serves ads on this wiki, strives to eliminate click fraud through its "Infinite Auction" system. Another problem for web-based advertising are pop-up and ad blockers, which interestingly prevents you from viewing the above site properly.
Another form of ads is through pop-up windows. Those are hated more than Uwe Boll. They not only are intrusive, waste memory and occupy task bar space (depending on your operating system UI and/or internet browser), but some have the gall to disguise themselves as alerts from your system.
Some ads take it to the extreme of being outright evil and use a security hole in your browser to install malware on your system. If you're viewing it on anything other than Windows, you can foil the attempt of malice because they're asking you to download a Windows executable (though, honestly, even if you're using Windows, programs generally don't install themselves, and even if they did, you usually know about it beforehand.) Even more obviously is if they are showing the GUI for Windows XP while you are running on Windows 7, Windows 10, MacOS, Linux, and, more amusingly, Smartphones.
Another form of advertising is the in-page pop-up, or lightbox. These are far more annoying, as they take over the entire webpage until you can close them, a more difficult task than strictly necessary since they don't use your operating system's standard GUI.
By far the most annoying of all these ads are be the flashing pop-up ads. These appear in their own web page, disappear into the background for a second, and then reappear. The hope is that they are hidden in the brief moment of time you are heading to mouse the "X" button in the corner of the browser window so instead of closing the ad, you don't react fast enough to the flash and close the browser windows you are working in, leaving the ad open instead. Safe to say, people generally find these annoying.
Internet advertisements are also one of the cutting-edge areas of research for annoying users with multimedia software, pioneering techniques such as embedded frames, animated GIF banners, blink tags, auto-playing audio and video ads, which are almost always powered by Flash, or some poorly made java-based video player. Needless to say, this tends to eat up enormous amounts of bandwidth and cause compatibility problems (up to and including instant crashes) on pages that would otherwise present only meager burdens on the reader's system. Most people hate these type of ads.
This site has some interesting pages about ads, and how we resent them or ignore them. If its author were an annoyologist, he would deserve big bucks.