Since that webservice is a hot topik on the forum, I decided to write a guideline to understand what you can do when you need to use webservice from iOS (so iphone or ipad or ipod, etc...)
Section 1: About Webservice
Section 2: REST webservice
Section 3: SOAP Webservice
Section 4: HTTP Request
Section 5: Parse XML
Section 6: Parse JSON
In the first Section we will see some basic things about webservice, in Section 2 and 3 we will see 2 important architectures of webservice and how we can use these. In section 4 we will see some example to communicate with webservice, and in section 5 and 6 we will see how we can use what the webservice return.
Section 1: About Webservice
Basically is a server that expose methods, you can call these methods with an input and you will receive an output. A webservice can be written in any language (php, C#, VB.Net, Java, Python, etc...).
Really webservices are much more than that, but this is not the place to study the argument in dept.
In iOS SDK there isn't a framework for consume webservice, so you need to create something from scratch, this can be "easily" done because webservice generally support HTTP + XML or JSON (in some case HTTP is not enabled for default, you need to enable manually yourself).
WSDL is a file that describe in depth the webservice (so contain a list of methods, with detail for their input and output, more other stuff)
XML Is a standard way to represent data in textual format.
JSON is another standard way to represent data in textual format.
SOAP and REST are 2 architectures of webservice. The first use SOAP messages (subset of XML) to communicate with webservice, so you will use http requests to send\receive SOAP messages to communicate with the webservice. With REST you can use http requests using GET (or POST, PUT, DELETE) to call a method of webservice, and XML or JSON as input and output.
There are many other things to know about webservices, start reading from the link posted above if you want have a good explanation of the topik.
Usefull links (tutorials + other stuff):
iPhone SDK: First Steps With JSON Data Using the Twitter API
Tutorial: JSON Over HTTP On The iPhone | Mobile Orchard
JSON Framework for iPhone (Part 2)
iPhone Programming Tutorial ? Intro to SOAP Web Services | iPhone Programming Tutorials
Consuming XML Web Services in iPhone Applications
NOTE1: This tutorial is not intended to be a perfect explanation of webservice (so some term and phrase can be not 100% exact), it is just a guideline to consume webservice with iOS SDK.
NOTE2: My english is not so good, so if you find some error, tell me privately, i will fix it.
NOTE3: Feel free to add some usefull informations (links, approaches, utility, etc...), I will try to keep updated the thread with all options possible.