One of the main differences the more average user might care about is the 32 Bit operating system has a maximum amount of RAM or memory of around 3.5gigabytes, where as the 64 bit system can go up to 4gb or more in RAM.
When your friend said if you don't know what 64bit is, stick with 32, he probably means that usually people who actually NEED or use that much computing power on average will know the difference. If you are planning on playing the latest and most advanced computer games, or doing video editing, you might want the extra RAM. If your just going to browse the internet and send emails and look at pictures, you don't need the 64 bit system.
