Canvas Data provides access to your Canvas data. The no-extra-charge (Canvas calls it free) version comes as flat files that you can download and then use.
The Canvas Data Portal website includes a description of the data that is available by describing the schema.
The web interface provides you with a list of download links, which is great if you want one or two files and you want them through a machine that has a graphical interface available.
I suspect that many people will want access to all of their data and they will want it without having to sit there and click all (as of November 2015) 50 files one-by-one. That's where the Canvas Data API comes in.
The PHP folder contains code that can be used with PHP to access the Canvas Data API
The PERL folder contains code that can be used with PERL to access the Canvas Data API.