What to use the model for?

[eluser]BrianDHall[/eluser]
Don't forget about the concept of libraries - there is a library for curl itself in CI, for instance.

The above link is a good one. In a really small nutshell, models are concerned with your applications data and certain manipulations of it. So you might have a billing Model that would fully encapsulate the table you use for your billing information, and have functions such as adding a payment, reversing a payment, getting a clients payment history, etc.

For things like curl and parsing YQL it seems like it fits the use of libraries, while the Model might invoke the library to get the data from YQL it needs - and then the model handles its storing and manipulations and such.