If you think XML is too complex and has too much overhead for your needs, or simply your backend does not support it, then you should check out a JSON parser.

Here is a good tutorial with parsing some Flickr JSON messages:
http://iphonedevelopertips.com/networking/iphone-json-flickr-tutorial-part-1.html

The JSON parser used in the tutorial, can be obtained freely from here:
https://github.com/stig/json-framework

Update
A faster JSON parser is also available with same functionality: YAJL
(Be sure to download the YAJLiOS framework when clicking the Download on GitHub site)