Fixtures are data that's loaded into the model (and ultimately the database) to initialize the state of the project. Some fixtures are to initialize the project itself. Others are used to set up test cases.
Depending on whether you are using Propel or Doctrine, you may need to organize your fixtures differently. They are Yaml versions of data. They need to contain data appropriate for their model definitions. The top level is the entity name, each level below that is a named instance. Named instances can be references elsewhere via their name. A subset of php can be used to pre-process these files, allowing you to populate large data sets.
Propel (currently) requires the reference data to be defined before it's referenced. So, file names beginning numerically is commonly seen.
data from a fixture file from from Jobeet(Doctrine):
JobeetJob: job_sensio_labs: JobeetCategory: programming type: full-time company: Sensio Labs logo: /uploads/jobs/sensio_labs.png url: http://www.sensiolabs.com/ position: Web Developer location: Paris, France description: | You've already developed websites with symfony and you want to work with Open-Source technologies. You have a minimum of 3 years experience in web development with PHP or Java and you wish to participate to development of Web 2.0 sites using the best frameworks available. how_to_apply: | Send your resume to fabien.potencier [at] sensio.com is_public: true is_activated: true token: job_sensio_labs email: email@example.com expires_at: '2008-12-25'