A persona is a profile that describes a type of user you have. A business will typically have several that describe the characteristics of different types of people. They are more of a second hand form of evidence, because they are usually distilled from a piece of user research like customer interviews.
However it is common to be brought onto a project and to be given personas as the output of previous user research. At this point they can become your main form of evidence about an audience. This often happens if you are working on a budget and you don't have the time to do a new piece of research.
If they are put together well they can tell you a lot about the people you should be designing for, and should be combined with your other sources of evidence like audience analytics data. On the other hand, if they're missing key pieces of information or they are getting old, you’ll need to use them carefully.
Sometimes they might not be called personas, but could instead be described as 'segments' or ‘customer groups'. Either way, they are usually pretty similar summaries of the types of people who use your product or service.
In this piece I won't go too much into how to create personas as there are plenty of articles out there that explain this (see below). Instead I'm more interested in how to use them as a piece of evidence for gaining insight and helping you design.
First you should check if your personas contain enough information: they should include the person's demographics (age, location, etc), their behaviours (particularly around technology); their motivations/goals; and their fears. It's important to have those fears and motivations in there for you to really be able to understand why people want your product or service. A good piece of user research should have probed this.
It’s good to critique them. Go through the information provided and see how it tallies with any other evidence you have—take notes or highlight sections. For example, demographic information should match what you can see on web analytics, and motivations should chime with survey responses. Where something seems wildly different it's worth flagging it and questioning it with your client or others on your team.
There should be more than one persona, so make sure to study them all and understand the differences. Whilst a few products are laser-focussed for only one type of user, in reality most will appeal to a few different types of people.
One of the advantages of personas is they remind you that your audience consists of people with varying motivations. This helps you avoid the trap of designing for a non-existent perfect customer, or trying to design for 'everyone', which can mean you end up creating something for no-one.
Once you have understood the personas you should be able to create some key goals for your designs to meet to satisfy these user types.
Whilst I do promote 'outline personas' as a quick way of summarising your audience analytics data, they aren't a replacement for full personas. They give you a sense of who your audience are and help you understand the different groups out there. They can be very helpful for knowing who to recruit for research or user tests and can tie groups of users to online behaviour. But without some qualitative research you'll always miss the important understanding of why people do what they do.
Be careful of excess demographic information in personas. It's useful to place people in your mind with an age, location, and job but any more than this and you can run the risk of drawing irrelevant conclusions.
Unless it's important to your product, ask yourself does it matter that your users have three dogs, speak Swedish, and exercise at 5am? As humans we can naturally lead ourselves to making stories out of these things and creating connections, even when a person's pets has nothing to do with them choosing a financial service (for example).
Watch out for being given too many personas or customer segments. If research was too broad a client can end up with ten or more personas, which can make it very hard to not design generically and create lots of features to satisfy everybody.
Also over time a company may have amassed several rounds of personas. You should try and understand who the current relevant ones are and trim it no more than four or five, which summarise the majority of your users.
Finally, do remember if you are given personas to work with, that they are second hand and rarely replace you actually talking to or watching users. If you can, try and speak to the people who created them and probe them on the details. It's usually worth you trying to do some of your own first hand research alongside to compliment or disprove them.
Personas take no specialist software to create. Whilst I'm sure specific tools exist, they can just be text documents or simply formatted presentations, outputted as PDFs.
The process of reading through and understanding personas should be quite a quick one—an hour or two.