The PCB is the physical manifestation of your design and represents your ultimate goal: To make a PCB.
A typical PCB
The PCB showing the PCB filled and physical parts on the top of the PCB
The PCB viewed in 3-D
It consists of a PCB border, parts and electrical connections.
A PCB border
The border can have any number of cutouts and round hole. Cutouts are usually created with a CNC cutter and are expensive. Round holes are simply drill out.
Parts
You place the footprints for your physical parts inside the border and usually not covering holes and cutouts. However, sometimes you may place a part over a cutout/hole so design reasons.
Parts can be placed on the top or the bottom of the PCB.
The parts are connected together either by electrical copper tracks or via internal layers of solid copper called signal/power planes.
Copper Tracks (electrical connections)
Tracks are parts of electrical PCB nets.
A Net is a collection of pads that need to be electrically connected together. DEX calculate a optimal unrouted non-cyclic* graph of unrouted track.
Nets map 1:1 with schematic nodes.
You manually route the unrouted tracks. That's the fun part. You can also use an auto-router.
*non-cyclic - does not form loops