Advertising is a communication whose   purpose is to inform potential customers about products and services and   how to obtain and use them. Many advertisements are also designed to   generate increased consumption of those products and services through   the creation and reinforcement of brand image and brand loyalty. For   these purposes advertisements often contain both factual information and   persuasive messages. Every major medium is used to deliver these   messages, including: television, radio, movies, magazines, newspapers,   video games, the Internet and billboards. Advertising is often placed by   an advertising agency on behalf of a company.
In short, Advertisement is : 
Information  for persuading and  motivating people so that it will attract them to  the service and the  thing that are offered or informed.
Function of advertisement :
Promotion 
Communication 
Information
In making an advertisement keep the following points :
1. Language of advertisement :
Using the correct or suitable words.
Using the interesting and suggestive expression.
Using positive expression 
Text of advertisement should be directed to the goals.
2. Content of advertisement :
Objective and honest 
Brief and clear
Not mocking to group or other producer.
Example of advertisement :
1.Mobile billboard advertising
Mobile  Billboards are flat-panel  campaign units in which their sole purpose  is to carry advertisements  along dedicated routes selected by clients  prior to the start of a  campaign. Mobile Billboard companies do not  typically carry third-party  cargo or freight. Mobile displays are used  for various situations in  metropolitan areas throughout the world,  including:
1.Target advertising 
2.One day,and long term campaigns 
3.Convention 
4.Sporting events 
5.Store openings or other similar promotional events 
6.Big advertisements from smaller companies 
2.Public service advertising
The  same advertising  techniques used to promote commercial goods and  services can be used to  inform, educate and motivate the public about  non-commercial issues,  such as AIDS, political ideology, energy  conservation, and global  warming.
3.Commercial advertising
Commercial  advertising media  can include wall paintings, billboards, street  furniture components,  printed flyers and rack cards, radio, cinema and  television ads, web  banners, mobile telephone screens, shopping carts,  web popups,  skywriting, bus stop benches,, magazines, newspapers,  posters, and the  backs of event tickets and supermarket receipts. Any  place an  "identified" sponsor pays to deliver their message through a  medium is  commercial advertising.
Covert  advertising is when a  product or brand is embedded in entertainment  and media. For example,  in a film, the main character can use an item  or other of a definite  brand, as in the movie Minority Report, where  Tom Cruise's character  John Anderton owns a phone with the Nokia logo  clearly written in the  top corner, or his watch engraved with the  Bulgari logo. Another example  of advertising in film is in I, Robot,  where main character played by  Will Smith mentions his Converse shoes  several times, calling them  "classics," because the film is set far in  the future. I, Robot and  Spaceballs also showcase futuristic cars with  the Audi and Mercedes-Benz  logos clearly displayed on the front of the  vehicles. Cadillac chose to  advertise in the movie The Matrix Reloaded,  which as a result contained  many scenes in which Cadillac cars were  used. Similarly, product  placement for Omega Watches, Ford, Vaio, and  cars are featured in recent  James Bond films, Casino Royale