Search engine marketing is the easiest way to get your website found on the search engine like Google, Yahoo or Bing. Ranking on the 1st page of search engine result page (SERPs) is not an easy task. Therefore, most site owners must advertise in search engine to have their websites found. When advertisement is on search engine, site owner is only charged when someone clicks the advertisement. This is known as pay per click (PPC) advertising. This form of online advertising is usually referred to as AdWords. If you advertise online, each click on your ad will lead to your website. People will find your ad by entering the keywords you choose to target.
For example, if you are a Web designer advertising for web design services, your keyword will then be "web design services". In this case you have bought a list of keywords in search engine that are related “design services”. When this happens, anyone who search keyword like “web design services” will find your advertisement on the search result page. When they click on your ad, it will take them to your website where you want them to go. Hopefully, they will buy from you through the information in the web page where you have led them.
People usually refer to search engine marketing as SEM in short. SEM services can be done through SEM agency, where the SEM consultant will advise you and do keywords research for your campaign.