SPSITE and SPWEB
In the programming model of the SPS, the two objects to be used first are SPSITE and SPWEB. Tell the program through these two objects you want to access which site on that server (virtual server). However, when you just learn the SPS programming object model, you always don't know the relationship between these two objects (because they are always used together), after a toss, I have finally have some understanding.
SPSITE is used to tell the user which server is opened, then, then return a site via spsite.allwebs ["sitem_name"].
SPSite is used to tell the user which site is opened.