Pen and paper
A "Pen", like its sister device the "Pencil", is a writing implement. Whereas a pencil commonly uses graphite to leave marks on a surface, a pen normally uses ink. Ink is available in a wide range of colors, though Black and Blue are most common. Pens use various means to deliver the ink to the surface, most notably ball point and felt tip. Oddly, no one uses nib pens anymore.
"Paper" is a flat writing surface normally created from wood pulp and sold in sheets or rolls. Character sheets, GM handouts, and reference sheets can be printed on paper. Pens and pencils can then be used to further dress up the paper by adding information or doodles by hand. Paper comes in different textures and thickness, examples being cardstock and toilet. Standard copy or computer paper is normally used for most gaming sheets. Toilet paper can be used to wipe your ass or wipe up your cum after you fap and/or shlick.
"Pen and Paper" tends to mean the gaming you do that doesn't involve you playing WoW. Computers are getting more and more common at the gaming table. So, one day, "pen and paper" will probably just be called "NonWoW gaming".