With more and more users surfing the web on their smartphones or tablets - not just their laptops/computers - it is even more important to have a mobile website. A great way to achieve this is to have a responsive website, which is thereby mobile friendly. A website is reponsive when it reformats/re-stylizes the webpage dynamically for web surfers based on the size of their screen - whether it's a laptop/computer, tablet, or smartphone. Mobile friendly means that your website is nice to anyone checking it out on their mobile device! In another words, they won't have to repeatedly zoom in/out, scroll to the sides, and they can click on links easily.
Having a complementary smartphone app is very beneficial especially for social networking and e-commerce websites that want to get the greatest interaction/user experience. With apps, users can use their mobile camera for example to upload photos/videos on the go easily, and also engage on your website with greatest ease on a platform built just for mobile devices.