There are actually quite a few differences between the two. However, SEOs in the Internet Marketing industry typically refer to each respective form of optimization as on-site and off-site.
SEO (Search Engine Optimization) is bisected into two separate parts: on-site and off-site optimization. With on-site optimization, you make adjustments ON your website that will benefit it’s rankings, traffic, etc. With off-site optimization, you do something FOR your website (on other sites) that helps boost rankings, generate traffic, etc.
On-site or On-page optimization consists of such things as editing the meta tags, heading tags, images, page title, URL structure and regularly adding relevant and unique content. This is something that you may want to discuss with your Webmaster or hire a team of professionals to handle all of the different aspects to ensure everything has been implemented correctly.
Some examples of off-site or off-page optimization are social bookmarking, forum posting, blog commenting, directory submissions, article submissions, press release submissions and posting regularly about your brand/market and your services on all of the various social media platforms (Facebook, Twitter, Tumblr, Pinterest, Instagram, etc.).
Online Marketing Philosophy
Internet users, and hence all people in general are becoming more and more “search-based” when it comes to finding answers to their questions or solutions to their problems. The vast majority of people actually make a conscious decision to go online to search for something that they want.
By optimizing your site to contain the keywords that your customers are searching for, and by making sure that your site is continuously updated with fresh and original content that contains said keywords, your website will start to show up in front of the right people at the right time.
Instead of YOU working hard trying to find THEM, it is exactly opposite! Furthermore, these visitors will be the most targeted and qualified leads a business could ever dream of having.