A web browser is the gateway to the internet for all. They have evolved from showing simple text to a very sophisticated form. Nowadays there are integrated chat applications through which you can rope in one of those black escorts and have a great time.
The W3C lays down the rules as to how a website mark up should be. If browsers comply with those standards it is easy for website builders to ensure that the website that they build will appear the same on all browsers. This is essential as different people prefer different web browsers for different reasons just for the same reason people like their coffees made differently.