There is plenty missing from so many websites these days. Proper grammar, correct spelling, etc. Oh sure, you have cutting edge eye catching killer graphics, and a (possibly) very coveted domain name. But if you hire someone with poor language skills (skillz?) to manage your website, it will usually show. And not in a good way.
You're = you are. The apostrophe stands for the missing letter (a, for those of you learning this for the first time and reading this with a puzzled expression).
Your = the possessive form of you. As in 'your website sucks'. Use it when you want to show ownership. Which you may not want to do if it's riddled with errors.
I will state again that I was not an English major, nor do I proclaim to know all the rules and such that apply to the language. So I may have gotten the definition of 'your' a bit wrong. However I stand by it as it will make sense to laymen such as myself!