in reply to How to make a secure website
You are interested in using "cookies" and "user authentication". Both of these are rich and complex topics. I suggest you use those as keywords to Super Search here on PM, or on google, and also check out some of merlyn's writings, such as Basic Cookie Management to help you get some background information and techniques.
There are quick answers to your questions, but getting a good grip on the big picture is priceless.