SEO or search engine optimization include strategies and techniques to help your website attain higher rankings in search engines such as Google, Yahoo or Bing.
SEM refers to search engine marketing, a detailed advertising plan to promote web traffic to your site, typically to increase leads or generate sales.
SMM means social media marketing, or the use of social platforms such as Facebook, Twitter, Google+, LinkedIn, and many others to spread your message and develop relationships with your followers.
PPC or pay-per-click is the primary online advertising medium. Literally, your ad does not cost until it is clicked, generating a visit to your website. PPC ads target active online shoppers and can be finely-matched to your product or service to deliver highly-qualified prospects to you. Search ads are the most common, but Display and Video PPC ads can be very efficient in building traffic and results.