A persona in UX Design is the characterisation of a user who represents a segment of your target audience. On a project you might create any number of personas to be representative of a range of user needs and desires. The solutions you design must answer these needs in order to deliver value to your target audience.
Typically Personas are created at the definition phase of a project to better understand the specific needs of your target audience. Personas are used as a reference throughout the project lifecycle to ensure that every decision is made in service of the personas needs that have been identified. If you are designing functionality that does not directly address a personas need then you should either not be designing it, or your personas are incorrect.
The details contained in personas should be formed from research with real/future users. It can be too easy to simply fabricate a set of personas to exactly match internal project requirements. Personas should also be updated occasionally as perceptions, opinions and needs can change with time.
You would typically have more than 1 persona, but beware of creating too many. If a small number of personas does not accurately represent your audience consider segmenting them into primary and secondary groupings, with most focus given to the primary.
Personas are typically created by a member of the design team responsible for requirements, research or user experience. We use personas on most projects and often help define the initial set of user stories that we might explore.
Understand your audience