Both are. On-page and off-page SEO are both important to get your site on the top results of search engines. Wouldn't it be great if you have quality content and a well optimized keywords, title, and meta tags to promote?
Web pages can survive fairly well by themselves without any off-site influence, provided they naturally outclass the linked-up fluff buy offering the search engine something indisputably unique, inherently important, exceptionally crafted with well optimized content. Link building SEO is for SEO weenies'. As a matter of fact, any attempt to artificially influence your off-site ranking factors is grey hat to black hat SEO and are practically all overly copy-catted techniques of little positive value generally and the only skill a person needs to be good at it is the quick abilities to copy/paste. Whooooooa. These guys are good (puking emoticon). Sorry for the rant folks. I feel better now.
On Page SEO is simply refers to the text and content on your web site pages. Basically editing your page and content. Meta Tag, Meta Title, Meta Description, Quality Content, Internal Links are few example of On Page SEO.
Off Page SEO can do off site to incraese your rankings is build up more links. More links will generally lead to better Google PageRank and better search engine ranking. SMO, Article Submission, Social Bookmarking, Forums, Article Submission, Blog Commenting are example for Off Page SEO.
... More links will generally lead to better Google PageRank and better search engine ranking ...
Higher PageRank (Google PR) doesn't mean squat in the results pages. A well-optimized PR0 (0 PageRank) web page with absolutely no incoming links can easily beat the fluffed-up mediocrities (PR 5-6) competing for the same keyphrase. I see this all the time and so can you if you look.
You are asking like water is important or air is important for life.
Couldn't say this better. Both are extremely important for good rankings. But in order to take the best benefit from off-page SEO, try doing the on-page work first. This is a general rule and you should always work on your on-page SEO first.