While services like OSH Park let you upload your Eagle CAD files directly for PCB manufacture, most other services, especially production runs, require the industry-standard Gerber files. Essentially a set of text files for each part of the board (ex: top copper, bottom silkscreen, bottom solder-mask, etc), generating Gerbers in the right format can be a bit tricky.
This tutorial walks you through this process, with a specific example of sending files to Seeed Studio’s excellent PCB service (no financial stake here – just like their service!). However, you could use these directions for most any fab house.
Special thanks to Luca Dentella’s post that helped me figure out this process.