At the moment, the majority of people browse the internet use Microsoft Internet Explorer (IE). This fact has lead to many sites being created that only work properly in IE. With a little care and experience sites can be made to work in all browsers, reaching 100% of web users.
Although IE currently has a near-monopoly in web browsers, this hasn't always been the case, and almost certainly won't be the case in the future. Netscape Navigator was a very popular browser in the early days of the web, and its latest incarnations as Mozilla and Firefox are gaining in popularity as more and more serious security flaws are found in the ageing IE code. The commercial Opera browser also has a strong following of users.
Writing standards compliant pages helps to ensure that all browsers show the page in a similar way.
A rough guide to current browser popularity can be found at w3schools.com.