USDA Hardiness Zones are based on the average minimum annual temperature. I live in "Zone 5" which means I can expect temps of -10F during the winter. You can look up the temps associated with the various zones in about a million places on the web, including (of course): wikipedia, which also has references to hardiness zones in a number of places around the world.
While annual minimum temperature is sometimes a useful bit of information, it is often useless:
- A question about the best time to plant cabbages in the spring doesn't care about how cold it's going to be in January. This will be localized, requiring fairly specific information about when the really hard frosts have tapered off. (We get frosts a fair bit later than people who live just 25 miles down the road.)
- A question about caring for plants in a warm area may care about how hot it gets in the summer time. Maybe it's ok to start lettuce in July in Fort Kent, ME, but you certainly wouldn't want to do it if you live in Dallas, TX.
- And rainfall: Phoenix and Tampa are both zone 9. But Tampa gets 45 inches of rain a year while Phoenix gets 8. Answers to the question about fast growing shade trees will be radically different -- who cares how cold it's going to get!
- And length of growing season. We have a short growing season (defined as time between frosts): roughly June 12 - Sep 7 (115 days). Last year we had snow on Mothers Day and again in mid-September! I put out my basil a week ago because it looked like it would stay warm and then we got a 38F night. There are annual vegetables that we can't grow outdoors because they won't mature in time. Topeka, Kansas is also in Zone 5, and they have a 175 day growing season -- that's an extra 60 days to ripen watermelon!
- And soil type (clay, loam, sandy) and pH (acid, neutral, alkaline), urban vs suburban vs rural, pests (e.g. we have deer but not pocket gophers) and countless other considerations.
The Zones are useful for figuring out if perennials are likely to survive the winter where you live. Otherwise I'm not sure it has that much value. There are just too many variables that any kind of categorization system like the Zones can really have much value for a site like this. If you though rainfall, temperature, and length of growing season were worthy of categories and should be applied as tags, you've just used up 3 out of 5 possible tags.
I'm a big fan of detailed questions that contain the information needed to answer the question, and comments on the question to elicit information that is missing. (If the asker knew enough to include all of the relevant information in the question, he'd maybe have already figured out the answer!)