Ever since the advent of Internet, advertising on the World Wide Web has been very popular. Many corporations, companies and business have taken advantage of this and you can see ads on any web pages you visit. Consumer can go to any search engine and type the keyword relating to what they are looking for and hit search and they will be provided with a huge list from which they can select. This is a very cost effective and time saving method of advertising. It has become really easy for any business to have a personalized website by which they can advertise, directly interact with the customer; provide details about their product and services. Regular newsletters, offers, discounts can be pasted on site to increase the interest of the website visitors. Since it’s easy to reach any kind of audience concerning any kind of business, the possibility of misuse is always there. Based on this Internet advertising is classified into two kinds of advertisement, legal online advertising and illegal online advertising. Legal online advertising includes online advertising directories, search engine advertising, e-mail advertising, and desktop advertising. Illegal advertising is more commonly know as spamming. This is usually done by altering some system settings with the help of external applications after which pop-ups are sent to a particular network or computer. The external applications are known as adware or spyware. Some of these are really harmful, the most famous being Trojans, which are very hard to uninstall and remove from the system. With the increase in technology, special effects are being used to make advertisement more interesting. Vivid colors, good page layout and lots of imagination is involved. Typically Adobe Flash is used to design advertisements these days. Depending on the technology being used to design advertisements can be classified into various categories. Banner ads are animations displayed on the website usually created in HTML or Eudora. There is a range of type and sizes of the ads. Trick banner ads are banner ads that have an extra functionality of dialog boxes and buttons and are displayed as an alert or error message. A pop-up is an advertisement displayed in a new window that covers up the active web page. A pop-under advertisement opens in another window that is under the active web page and can be seen after the present window is either closed or minimized. Interstitial ads are those that are displayed before directing over to the desired page. Wallpaper ads form the background of the web page. The ads that float on the screen are known as a floating ad. Polite ads download on a low pace without interrupting the normal functioning of the website. An ad that enlarges and changes the contents of the page being displayed is known as expanding ad. Advertisements which are displayed in a video form on a website is known as a video ad. There are many ways by which advertising slots can be purchased on Internet like CPM, CPV, CPC, CPA, CPL and CPO. CPM or cost per mil means that the advertiser is meant to pay for a particular number of people to whom the advertisement will be exposed. CPV or cost per visitor means that the advertiser is meant to pay for the people to whom the advertisement was delivered. CPC or cost per click means paying for the number of clicks made on the advertisement by the visitors. Although the advertisement is put up on the website, the amount is paid only after the visitor clicks on the URL of the advertisement. CPA or cost per action means that the advertisement publisher bares all the charges of advertisement, but he gets paid only if the visitor clicks on the advertisement and purchases a product or signs-up for a service. CPL or cost per lead is similar to CPA, only that the visitor doesn’t have to necessarily buy anything; he or she can simply apply to get regular newsletters and special offers. CPO or cost per order is where the advertiser pays each time an order is placed. Online advertisements cannot only be used to promote a product or service but in fact they can be used for purposes like promoting charity and spreading education.