Parsing a string of form data in Python

Question

I'm using Python to read some data that was originally submitted to a form and acquired from an API. The form data in its entirety exists as a string.

I don't recognize the format of this string, though, and I'm not sure how to properly read this data.

String is formatted like this:

 a:24:{s:9:"FirstName";s:5:"Myfirstname"; ... }

1. I don't recognize this format with the groupings of letter:number:. What is this string?
2. How can I parse this in Python?

Thank you!