It took me years to get my head around the concept of “unique visitors” to a website, as mentioned in your friendly local web analytics program (probably). Analytics guru Avinash Kaushik explains what they are, and what the different types you hear about mean, in Standard Metrics Revisited: Daily, Weekly, Monthly Unique Visitors from his Occam’s Razor blog. It’s a long article, but straightforwardly-explained throughout, so do bear with it, and you’ll be rewarded with an understanding of a crucial concept. Most importantly, it will explain why “Daily”, “Weekly” and “Monthly” Unique Visitors are a misleading statistic in most everyday use, which you should avoid.
Most importantly, you should never ask or answer the question “how many unique visitors does your site have?” unless both parties understand exactly what is meant by the term. When I used to manage the Pro-Talk websites, advertising agencies would ask us this question every day, and I’d always ask them to clarify the question before answering it. They rarely could – they just thought they were being clever, when in reality they were just showing their ignorance. They still do it, apparently – collecting completely incomparable data from various websites and then using it for comparative purposes. They’re not doing their clients any favours.