Every call to the must contain the version number. The latest version is v3.
When we update the API in a backward compatible way (common case), we don't change the version, but the documentation will be updated.
When we update the API in a way that is not backward compatible (rare case) we'll create a new version. The version you're using will remain unchanged.
Compatibility with v2
With the new version (v3) of the API we've changed almost everything to make it standard, faster and more reliable. Unfortunately it means it's not backward compatible and you'll have to update your code.
The documentation of the API v2 can be found here.
Updated about a year ago