Eagle CAD: Use schematic as part
You can use Eagle Design Block if you feel like the circuit part could be re-used multiple time.
But maybe you just want to copy some parts + connections from one schematics to an another.  In this case:

Open both schematics. You actually have to start Eagle twice.
Go the schematics, from which you want to copy stuff.
Click on group and select whatever you want to copy. It will be highligted.
Click on copy + right-click anywhere on the schematic -&gt; Copy: Group
Go to schematics where you want to place the selected part, click paste and it should appear. Place it with left click.

Please note, that the PCB routing design will not be copied this way. You have to route it again. If you want to copy that as well, use Design Block.