Nancy harden's questions

Hi,

I just want to have an inquiry. I bought iMac last week and my OS software is OS Snow Leopard. My officemates keep on bugging me to update my software to OS X Lion. Just wondering what's the difference between Snow Leopard and X Lion?

And if ever I update my software, will my files and applications which are downloaded stay or will they be deleted upon updating? Hope to hear something from you. Thanks!