What are signed cookies and why are they useful?

In some present开发者_StackOverflow中文版ation from EuroPython "signed cookies" have been mentioned. Since googling didn't help -- I haven't found anything besides "yes, they are cool, this is how you do it", most in context of Django -- maybe someone on StackOverflow has a good explanation for me.

So, what are signed cookies, why would I want to use them, how are the downsides?

---

A signed cookies provides a mechanism where you can trust the data in a cookie from a client.

The upsides are that you wouldn't necessarily have to store any session information on your server.

The downside is that you should keep the cookie data to a minimum.