Eagle is a software package that aids (CAD: Computer Aided Design) in the design of printed circuit boards (PCB).
Through hole, Surface Mounted Device (SMD), as well as mixed PCBs can be made. Through hole are components like 1/4 watt resistors and large capacitors (where you stick the wires through holes in the PCB). SMD are the small components that appear to be without wires. Square solder pads are used to connect the components to the printed wires of the PCB.
The freeware version is limited, but still very functional for Industrial Designers:
PCB dimensions 100 x 80 mm max
Only two layers can be used (top / bottom)
The schematic editor can only create one sheet
Top layer: blue (SMD components are normally connected and placed on the top layer)
Bottom layer: red
To finalize your design, you can add text to indicate top and bottom (handy if you etch your own design). Another necessity if you order PCBs is an anti-solder mask. This prevents solder to flow from one pad to the other.