Personas are a qualitative character crafted from User Research, to empathise with and design for a large group of users. They represent people who might use a site, brand, or product in a similar way.
Most companies cast their net wide, and try to please everyone. counter-intuitively, this is a bad idea! If you're trying to please everyone, chances are you're disappointing most people. After all think of it like archery: Making the target bigger doesn't improve your aim. You need to narrow your focus and rather than trying to please everyone, try to only please your largest group of customers. Once you've achieved that, then you can start to make adjustments to please the outliers, the edge-cases who don't fit the focused aim. You'll be much more successful that way.
With User Personas. These carefully crafted documents are the culmination of lots of research into your ideal customers. They are most effective because they put a personal human face on otherwise abstract data about customers. By thinking about the needs of a fictional persona, designers may be better able to infer what a real person might need. Such inference may assist with brainstorming, use case specification, and feature definition.
Personas are easy to communicate to development teams and allow stakeholders to understand customer data in an easy format. They also help managers understand their customers better, and often challenge preconceived impressions of customers with real data.
You can't measure these results with traditional methods. User Research finds the common information for your personas, and compiles that data into one human you can easily empathise with and design for. Once you've got enough commonalities in the data you can start to build up a persona around them. This means all those seemingly trivial pieces of information become useful for understanding and empathising with your users. Once you've built up empathy, you can appreciate all the pitfalls your users get into when using your website, and this will help you spot (and fix) all sorts of frustrating niggles.