SEO is the abbreviation for search engine optimisation and it is a set of strategies used to enhance effectiveness of web pages and to make it function at its best. SEO involves link building, web content, keywords, etc. SEM is the abbreviation for search engine marketing, which involves looking over your web pages carefully to ensure that your site is optimized to reach your targeted audience. SEM may involve PPC or pay per click marking, web site marketing, directory, and other forms of marketing, such as advertising in newspapers, television, etc. Strong and weak sides of SEO & SEM also involve:

When to use SEO and When SEM?
Is there a right and wrong time to use SEO and SEM?

At the beginning of your web design project you should start using SEO strategies to optimize your web pages and prepare them for SEM. SEM should be considered immediately after you have completed the design of your web pages and optimized them properly. Some people choose to market their web site after the Home and Contact pages are completed. This is ok, but you will get better results and save time on resubmitting your pages to Google, Yahoo and other engines by marketing your web site once it is completed. You can do some marketing ahead of the completion of your web site by telling others about your products or services. Word of mouth or creating “upcoming” services and products is ok, but provide the date in which you intend to start marketing your company.

Other questions include:
Which strategy will work for whom?

Like people all web pages are different. Some strategies may work for one website but may not work for others. No matter who you are it is important that you create dynamic web pages that are completely optimized and relevant to search queries of your intended audience. Some of the most effective SEO strategies including web content, link building, and URL submission; other strategies are used also, but the three SEO solutions are the main methods used to optimize web pages. You have to adhere to the trial-and-error methods to learn what SEO strategies work best for you.