Appendices (appendix) are used to present information that is related, but not critical to the main body of text. For example, large volumes of complex statistical analyses, technical information relating to equipment you've used, diagrams or completed questionnaires may be submitted as appendices.
In the same way you would reference a figure, table or plate, you must reference your appendices to enable a reader to cross reference, for example 'Of the 100 questionnaires sent out, 54 of them were completed and returned (see Appendix 1). The rate of return is consistent with ... '.
The appendix should be the last section of your work and comes after the references.