What is WebService ,Explanation with example

A web service can be thought of as a software application with the following designed to send and receive information from other software applications. For example -- you fill out your tax info on TurboTax.com and click send. Intuit's web service finds the IRS's web service, they talk to each other, and your tax info gets sent form Intuit to the IRS.
Other examples of a web service:

• When you log into your Android phone, you have to put in an email address and password. There's a server on the Interwebz somewhere that sits around waiting for login information. When the Google authentication service receives the login request, it looks up the info, and then tells the phone whether or not the request is legit.
• When someone swipes your credit card, the credit card machine sends a request over the Internet or a phone line to a web service that determines whether or not the transaction is legit.
• If you want an example with a little more lingo... Twitter built the first version of Twitter using Ruby on Rails. Twitter wants to let people post messages to Twitter, so they build a REST API that basically allows anyone to talk to Twitter's API if they use REST and follow certain instructions. Then FourSquare comes along and builds their check-in app using Scala and Lift. Scala doesn't know a damn thing about Ruby and vice versa, but they don't need to. Some dude uses FourSquare to check into Starubucks, the FourSquare web service receives the check-in from the dude's smartphone and then says "Hey Twitter, I'm sending you a tweet in that REST format you like." Twitter is just sitting there picking it's nose when a package arrives all wrapped in REST. Twitter opens it, says "Hey look, dude's tweet" and then posts a tweet in dude's account saying "Dude just checked-in to Starbucks".