Search Engine Optimisation(SEO) is crucial for the success of any business website. SEO will increase your traffic on your site. Also it will help to have a better place in organic search.
Search Engine Marketing(SEM) will help you to sell your products better than other competitors.
Social Media Marketing(SMM) will help you to find out what your costumers think about your products.You can get feedback about your website or your company.Your company have free publicity and also you can sell your products on social media networks.
All 3: SEO,SEMSMM go hand in hand.