This is a headache problem for website developers. In fact each browser parse the html contents with their own engine and shows them as per their own algorithm. The CSS reading and understanding also differ browser to browser. You can even see this difference in different versions of same browser (Like IE 7 and IE9). There are some basic standards follows by each browser. Try them (google.com for tips) to make your website look relatively compatible on different browsers. One example of tip is using of percentage for width of an element instead of using a fixed width so the page can automatically adjust to the size as per visitor's screen.
If you want to check your website in various browsers (and on different OS), the below URL will defiantly help you;
Another alternative of above URL is http://browsershots.org/ to help you seeing your webpage across multiple web browsers (an operating systems too).