Hi stevto,
The first thing you need to know is that search engines rank pages not sites. Each page has it's own score and may be returned in SERP if it is indexed.
The most important factor in ranking returned results is relevance to your keyword. The more relevant the search engine's algorithm finds your page the higher it will rank in the index.
When two pages have roughly the same relevance they prioritize by authority. Authority is influenced by other related sites that link to you. The more Authority those linking pages have for the searched keyword the more that passes to your page.
PR or PageRank is simply a score that measures the relative link popularity a page has compared to all other pages on the web. It's a quantitative number NOT a qualitative number so it has very little influence in
SEO. When all other factors (relevance and authorty) are equal, the
PR of a page may be the tie breaker, but will not out weigh authority or relevance.
So, as you can see, the key thing to focus on is relevance, followed by authority. I would recommend that you check out the
SEO Guide that Google has put out, it's an excellent primer from a reliable source.