If you have two PC's in the house connected on a network router so each has net access.... How can you get the PC's to recgnise each other?

In as afar as, if I'm upstairs on one PC and type up a document and want to print it, how can I send it to the downstairs PC (which is connected to the printer)

Or just send one file from one PC (say an MP3 or picture etc) to the other so I don't have to burn a data disc everytime I want to move something that is too big for floppy?

I will get one of them USB memory sticks at some point but interested to know how to do this