Eagle is a full-featured electronics schematic capture and CAD program. Schematic capture means that schematics can be entered, edited, modified and printed. The schematic files can be viewed on any computer running Eagle. Unfortunately, there is no industry standard format allowing other schematic capture programs to view Eagle's schematics, all schematic capture programs' formats are proprietary. CAD (computer aided design) in this context means that Eagle can take the schematic and make the necessary files for a "board house" to make a PCB (printed circuit board). Fortunately, there is a standard format for boards called gerbers and Eagle does follow that standard, antiquated as it is (something to do with spinning slotted wheels, check Wiki).
Eagle is not PADs. It does not link parts to a central data base. It does not roll over and do tricks. For 10% or less the price of a fancy CAD package, Eagle does 98% of it. Eagle's scripting language can do almost anything. Many scripts are available online and with patience you can write your own. If you want a full-blown package with all of the bells and whistles and you are willing to spend a lot of time learning its nuances and have a good budget then Eagle is not for you. I have used both PADs for years (from DOS versions to full environments) and am very happy with Eagle. A Ferrari in traffic is no faster than a Volkswagen, it just looks better.
The freeware Eagle has limitations: single sheet schematics, PCBs top and bottom layers only, 80 x 100mm max (half-Eurocard). The freeware version can view work produced by the licensed version but can only edit the schematic and layout within its restricted area as described above. Check the Eagle website (North American site) for licensing costs, educational version if applicable. The documentation that comes with the registered version is adequate if a little thin, online forum support is very good. All in all, CAD is a time consuming, complex and extremely precise process and Eagle allows the job to be done well.
One thing I would like changed, Mr. Eagle: all schematic and board files in ASCII so can manipulate and check them through other methods. Now, where did I use that ADCMP55x? Can't do a text search through all projects, have to slog through opening and searching each project. Is there more than one ground used in this project? Can't do an ASCII search. Complaints finished.
Download (North American site) the appropriate version from Eagle. Register it if you need larger boards or more than two layers or just received a big grant.
In the Control Panel - Options - Directories..., more than one path can be shown (e.g., link to our Quantum Optics library) by separating them with a semicolon. The first directory shown will be the one listed when a directory is required (e.g. script path "C:\tmp;C:\Eagle\scr", only the C:\tmp list will be shown with the command "scr").
The tutorial is quite good, if a bit thin. This is a complex program. At first glance, it is daunting. Like bicycling and quantum mechanics, once mastered, it is easy. When started, up to three windows appear: the project's Control Panel (oversees all aspects of the project but does nothing in itself), the Schematic (where you draw out the schematic) and the PCB Board (if applicable, shows the present version of what the board will look like).
In a nutshell ("well, basically"), start with a sample schematic in the Schematic window. Components are represented by symbols (but also associated by their PCB packages which you can probably ignore for now). Eagle can use a command line to edit the schematic. This said, there are five ways to edit with identical effect: type the command, select it from the menu, click the tool bar, click the command text (not shown in screen capture below), or click the command's icon (left side of screen capture below). For example, to move a part, select or type Move (just the first three letters "mov" are necessary, case insensitive), left click once on the part, relocate it then left click again to drop it. It stays in Move mode until another command is selected, which is guaranteed to catch you a few times. Another example: to connect a wire, select or type 'wir[e]', left click once to start the wire, left click as many times as necessary to add corners, then hit the ESCape key or double click or click on a component pin to end that wire. Be warned that you must select or type Junction and then click one in to actually join intersecting wires if they do not have a dot on the junction.
Where Eagle gets complex is in adding parts. Lucky you if the part exists, it can be located in a library then added. However, new parts have to have symbols, decals and outlines made for them first. A bit of practice and it can be done easily. Our Quantum Optics library (right click and copy to your Eagle library directory) is preset for all of the parts used here.