There are several articles online that explain Page Rank:

From Alvit.de…

Google PageRank (one word) is Google’s measure of the relative importance of a Web page on the Internet. The numbers rank from 0 to 10. The higher the number, the stronger the PageRank.

From Google…

PageRank reflects our view of the importance of web pages by considering more than 500 million variables and 2 billion terms. Pages that we believe are important pages receive a higher PageRank and are more likely to appear at the top of the search results.

PageRank also considers the importance of each page that casts a vote, as votes from some pages are considered to have greater value, thus giving the linked page greater value. We have always taken a pragmatic approach to help improve search quality and create useful products, and our technology uses the collective intelligence of the web to determine a page’s importance.

From GoogleGuide.com…

In other words, Google conducts “elections” in which each web page casts votes for web pages with hyperlinks to those pages. But unlike a democracy a page can have more than one vote and links from pages with high PageRank are given more weight (according to their ranking) and thus help to improve the targets’ PageRank.

From SelfSEO.com…

The facts that we do know about page rank are; 

1. One-way inbound links from websites with topics that are related to your website’s topic will help you gain a higher page rank. 

2. Other one-way inbound links from pages with high page rank but unrelated topics do help a little, but not nearly as much. 

3. The number of links outbound from the website that links to you also determines the value of the link. A related website with 10 outbound links that links to you is much better than a related website with 100 outbound links that link to you.

Also important to keep in mind that search engines crawl and index webpages not websites, that is why your page rank may vary from page to page within your website. 

Good internal linking and navigation can help distribute page rank well throughout your website. Your index page is the most important page Google indexes and your other pages will inherit some page rank from your index page because it is linked from there. 

I have one website that is a PR4 on the main page but contains several PR5 and PR6 webpages in the interior of the website proving that Google ranks webpages individually. 

When you are doing your link building campaign, do it for more than just the index page. Put links out there for your interior pages as well. 

Webmasters and individuals will link to quality content and quality content is the single best way to higher page rank and search engine listings. Content is still king. Links are not king. 

People link to articles and content that is original and well-written and that contain quality information not found elsewhere. Google also ranks webpages higher in listings that have quality content. That’s why it is still the best SEO method there is.