Wordtracker & Overture

Wordtracker & Overture
Currently, the two most popular sources of keyword data are Wordtracker, whose statistics come primarily from use of the meta-search engine Dogpile (which has ~1% of the share of searches performed online) and Overture (recently re-branded as Yahoo! Search Marketing), which offers data collected from searches performed on Yahoo!'s engine (with a 22-28% share). While neither's data is flawless or entirely accurate, both provide good methods for measuring comparative numbers. For example, while Overture and Wordtracker may disagree on numbers and say that "red bicycles" gets 240 vs. 380 searches per day (across all engines), both will generally indicate that this is a more popular term than "scarlet bicycles", "maroon bicycles", or even "blue bicycles."
In Wordtracker, which provides more detail but has a considerably smaller share of data, terms and phrases are separated by capitalization, plurality, and word ordering. In the Overture tool, multiple search phrases are combined. For example, Wordtracker would independently show numbers for "car loans", "Car Loans", "car loan", and "cars Loan", whereas Overture would give a single number that encompasses all of these. The granularity of data can be more useful for analyzing searches that may result in unique results pages (plurals often do and different word orders almost always do), but capitalization is of less consequence as the search engines don't deliver different results based on capitalization.
Remember that Wordtracker and Overture are both useful tools for relative keyword data, but can be highly inaccurate when compared to the actual number of searches performed. In other words, use the tools to select which terms to target, but don't rely on them for predicting the amount of traffic you can achieve. If your goal is estimating traffic numbers, use programs like Googles Adwords and Yahoo! Search Marketing to test the number of impressions a particular term/phrase gets.
Source: seomoz.org