A Web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards.
From W3C Web Services Architecture 
Technical details will be later explained in the documentation. What comes out is that web services provide a standard means of interoperating between different software applications. Each of these applications may run on a variety of platforms and/or frameworks providing a set of functionalities. The main concern is about interoperability between services.
- A service provider publishes a service contract that exposes the public functions (operations) it is able to perform and thus service consumers can use.
- Both service providers and service consumers features concrete softwares that send and receive messages according to the informations contained in the service contract they agreed before the communication.
- Basic Web services specifications define the standard way of publishing a service contract and communicating.
- Web services stacks (like JBossWS) conform to these specifications providing software layers to developers who want to either implement a service provider or service consumer. This way they almost only need to develop their own business logic in their preferred way, without dealing with the low-level details of message exchanges and so on.
Enterprise systems communication may benefit from a wise adoption of WS technologies. Exposing well designed contracts allows developers to extract an abstract view of their service capabilities. Considering the standardized way contracts are written, this definitely helps communication with third-party systems and eventually support business-to-business integration. No more agreement required on vendor specific implementation details, home-brew communication protocol or custom per-customer settings. Everything is clear and standardized in the contract the provider and consumer agree on. Of course this also reduces the dependencies between implementations allowing other consumers to easily use the provided service without major changes.
Enterprise system may benefit from web service technologies also for internal heterogeneous subsystems communication. As a matter of fact their interoperability boosts service reuse and composition. No more need to rewrite whole functionalities only because they were developed by another enterprise department using another software language.
Needless to say that web services are not the solution for every software system communication.
Nowadays they are meant to be used for loosely-coupled coarse-grained communication, for message (document) exchange. Moreover during the last years a lot of specifications (WS-*) were discussed and finally approved to standardize ws-related advanced aspects including reliable messaging, message-level security, cross-service transactions, etc. Finally web service specifications also include notion of registries to collect service contract references, to easily discover service implementations, etc.
This all means that the web services technology platform suits complex enterprise communication and is not simply the latest way of doing remote procedure calls.
Contracts carry technical constraints and requirements of the exposed service as well as information about data to be exchange to interact with the service. They comprise technical descriptions and optional non-technical documents. The latter might include human readable description of the service and the business process it is part of as well as service level agreement / quality of provided service information.
Service description is mainly provided using the standard Web Service Description Language (WSDL). Practically speaking this means one or more XML files containing information including the service location (endpoint address), the service functionalities (operations), the input/output messages involved in the communication and the business data structure. The latter is basically one or more XML Schema definition. Moreover recent specifications (like WS-Policy) allow for more advanced service capabilities to be stated in the contract through WSDL extensions.
Web service stacks like JBossWS usually have tools to both generate and consume technical contracts. This helps ensuring also from a practical point of view that owners of service producer (server) and consumer (client) only need contracts to establish the communication.
One of the main concerns about service contracts is the way they're obtained.
As previously said, tools allow developers to automatically generate WSDL contract files given their service implementation. Advantages and disadvantage of this delivery process include:
- Developers do not have to deal with contracts by hand thus deep knowledge of WSDL and XML is not required.
- Less effort and time required for services to be developed and go live to production environment.
- Contracts usually need frequent maintenance, refactoring and versioning.
Developers may write contracts first instead. This usually implies an initial collaboration of architects and business analysts to define a conceptual service design together.
- Services with contracts obtained this way may easily cooperate in a service oriented architecture
- More effort and time required for web service project start-up
- Deep knowledge of WSDL and related technology required
- Contracts tend to have longer lifespans and usually require less maintenance.
Messages' content is described in the wsdl contract. The WSDL file also states the transport protocol to be used for the transmission; the most common one is of course HTTP, however JMS, SMTP and other ones are allowed.
The above mentioned specifications are quite common nowadays in the IT industry and many enterprise have been using them since years.
However a real added value to the web service platform comes from a lot of recent additional specifications. These cover features that are really relevant to deliver mission critical enterprise services. For example some of the most important agreements major vendors came to are those on security (WS-Security) and reliable messaging (WS-Reliable Messaging).