At first use, adding components to a schematic is often frustrating. The libraries available on this site are organized to make this easier for DIY pedal makers. This page describes the organization as it appears in Eagle in more detail.
The Eagle “Add” command opens a window like the one below. The libraries that are in use are listed by name alphabetically. The “Description” column to the right shows the title of each library’s description.
There are also three window panes on the far right. The bottom pane shows the full description available for the selected library: the description title and any additional text.
These gm libraries give summary information about the values of the components in each library. For example, the ceramic capacitor library description shown here contains capacitors in the 1 – 1000pF range.
Expanding a library entry one level gives a window like this one. The entries are names for component packages, the size and shape of a component. Because schematics rarely refer to the packages of a component, this can be confusing for beginners looking for a component value like 47pF for a capacitor or 100K for a resistor.
In the example above, the package is called “050X030_025″ because the package is 5mm by 3mm with 2.5mm lead spacing. The description explains that the capacitors in this package come in values from 47pF to 120pF. So given that you are looking for a ceramic capacitor in the 1pF to 1000pF range, you can find the appropriate package quickly.
In the Eagle schematic layout editor (as in many other layout editors), you place a component package and then assign a value. This makes sense because a component usually comes in fewer packages than in various values. Also, because components vary along such other dimensions as voltage rating and manufacturer, a specific capacitance (like 47pF) or resistance (like 100K) comes in many different packages.
Expanding a library entry an additional level gives a window like this one. You can see both the schematic image and the circuit board image in the top right-hand panes.
In this example, the 5mm by 3mm package has a 0.1″ lead spacing for the circuit board image. If we just put the leads through the holes of a perfboard then we will have 0.1″ pad spacing on the solder side of the board. The other entries for this package give 0.2″, 0.3″, and 0.4″ pad spacing. These different versions matter only for circuit board layout. For schematic layout, which often precedes circuit board layout, any pad spacing will do.
I usually pick the smallest lead spacing, when I am paying attention. 
This image shows what a package with 0.4″ pad spacing looks like. In the circuit board image on the far right, the leads of the capacitor are bent out from underneath the capacitor to reach holes 0.4″ apart.
If you lay out your schematic first, then you can also use Eagle to layout your circuit board using the information in the schematic. You can choose different pad spacing for a component during the board layout design.
Laying out your circuit board based upon the schematic you create in Eagle can be helpful. Many people enter a circuit more accurately as a schematic than as a draft circuit board design. Eagle’s circuit layout editor displays the correct connections among components as described by the schematic layout. It also flags components that are incorrectly connected.