You can export your quizzes using the Import/Export/Copy Components tool, which generates a Zip file leveraging the IMS CP and QTI formats (http://www.imsglobal.org/question/) as a baseline, with some custom D2L fields thrown in. The resulting output may be a little daunting, but you can additionally import in the same format, and this format (unlike our CSV supported format) maintains the categories/sections of questions.
As an alternative, you can import in CSV. The format for that is available as a sample from the Question Library. Choose Import -> Import Source: 'From a Desire2Learn Text Format File' and a link to sample is provided.
An additional tool you may be interested in is the Question Importer Tool (https://academictech.doit.wisc.edu/help/learnuw-quiz-importer) which generates the CSV format, and provides an alternative authoring environment.
If you are looking to export your quiz results, that's available from Statistics, and there is an option to export results via CSV from there.