Off page SEO services deals with everything that is not on your website. Things like Meta tags, error messages, keywords, image alt image text, and so on are all on page SEO matters. On page SEO services will include things like link building, directory submissions, content optimization, blog posts, press releases, and on page directories.
All these techniques help you rank well on the search engines for relevant terms.
On the other hand, off page SEO services deals with things that are visible to users. This includes things like external links, title tags, outbound links, Meta tags, image and images, content descriptions, backlinks, and so on. There are many benefits of outsourcing this kind of SEO work to a professional SEO company.
Off page SEO is one way to benefit from their experience and know-how in order to rank well on the search engine results pages for relevant keywords.
Apart from helping you rank on Google and Yahoo! for targeted keywords, an SEO expert can also help you optimize your website for the search engine results pages for non-targeted keywords. The process of optimizing a website for the search engines is called 'SEM' or 'Search Engine Marketing'.