Google Insights offers a nice way to compare two terms to see their relative search frequency. For example, the chart below shows that black cats are much more popular than brown cats when it comes to Google searches:
Notice that the interest in black cats seems to peak around Halloween each year.
This is all nice, but it is all relative. We still have no idea how many searches are run for black cats each month. However, if we can find a single term were we know how many searches are performed each month, we can use that to figure out the approximate search frequency of any other term.
Google Insights is particularly useful if you are trying to target a specific geographic area. There are language differences that will influence what search terms people use. It is worth comparing a number of different near synonyms for a particular term to see how most people are searching in the particular geographic area you wish to target.