Think of a website as a research paper. The way that a research paper can be determined as important is if many other research papers refer to it as a citation in their reference list. Google's PageRank algorithm works largely in the same way. A webpage's importance (based on a scale of 1 to 10) is computed by working out how many 'back links' (websites linking to a page) there are and the importance of each of these links. If a lot of important sites link to a site then it is regarded as important.
Google's toolbar (available here) includes an icon that shows a PageRank value from 0-10.
The academic justification of the rank is "assume there is a 'random surfer' who is given a web page at random and keeps clicking on links, never hitting 'back' but eventually gets bored and starts on another random page. The probability that the random surfer visits a page is its PageRank." In effect, this means that some links are more valuable than others, as a higher PageRank page is more likely to be reached by the random surfer.
So, in order to boost your importance to search engines, you need to get as many sites that are similar to yours to link to you. Quality of links is however more important than quantity.
Forget about 'Free For All Links' or 'Link Farms' - Google considers them spamming and may penalise your site. Do a search for similar sites on Google and start trying to get links on the sites that are shown, in all probability they have the high PageRank score that'll help you most.
You can see the PageRank that Google has allocated to you by downloading the Google toolbar in your browser. Since PageRank's inception in the late '90s, more subtle and sophisticated link analysis systems have taken the place of PageRank. Thus, in the modern era of SEO, the PageRank measurement in Google's toolbar, directory, or through sites that query the service is of limited value. Pages with PR8 can be found ranked 20-30 positions below pages with a PR3 or PR4. In addition, the toolbar numbers are updated only every 3-6 months by Google, making the values even less useful. Rather than focusing on PageRank, it's important to think holistically about a link's worth.