When designing a User Interface, whether it is for a website or an application, there are many rules that can help you evaluate if the design will work or not. Using these rules of course isn't mandatory, but the principles behind it are proven. So it's a good thing to know them and use them especially when you are stuck on a design. Or when you are in doubt of what would work best. Jon Yablonski created a website, Laws of UX, where he brings together the most common principles.
If you rather watch than read, check out this video from Joseph Angelo Todaro where he explains them all in 10 minutes.
I'm sure you will find some usefull tips there. So enjoy, and may the source be with you!