I'm trying to teach myself a concept by writing a script.  Basically, I'm trying to write a Python script that, given a few keywords, will crawl web pages until it finds the data I need.  For example, say I want to find a list of venemous snakes that live in the US.  I might run my script with the keywords list,venemous,snakes,US, and I want to be able to trust with at least 80% certainty that it will return a list of snakes in the US.
I already know how to implement the web spider part, I just want to learn how I can determine a web page's relevancy without knowing a single thing about the page's structure.  I have researched web scraping techniques but they all seem to assume knowledge of the page's html tag structure.  Is there a certain algorithm out there that would allow me to pull data from the page and determine its relevancy?
Any pointers would be greatly appreciated.  I am using Python with urllib and BeautifulSoup.
You're basically asking "how do I write a search engine."  This is... not trivial.
The right way to do this is to just use Google's (or Bing's, or Yahoo!'s, or...) search API and show the top n results.  But if you're just working on a personal project to teach yourself some concepts (not sure which ones those would be exactly though), then here are a few suggestions:

search the text content of the appropriate tags (<p>, <div>, and so forth) for relevant keywords (duh)
use the relevant keywords to check for the presence of tags that might contain what you're looking for.  For example, if you're looking for a list of things, then a page containing <ul> or <ol> or even <table> might be a good candidate
build a synonym dictionary and search each page for synonyms of your keywords too.  Limiting yourself to "US" might mean an artificially low ranking for a page containing just "America"
keep a list of words which are not in your set of keywords and give a higher ranking to pages which contain the most of them.  These pages are (arguably) more likely to contain the answer you're looking for

good luck (you'll need it)!
Web scraping without knowledge of page structure