This circuit features a part I only learned about recently: a PTC resettable fuse. The PTC standes for Positive Temperature Coefficient. In brief these parts are designed to melt and break the connection like any fuse if they are getting too much current. Once the current goes away, however, after some time they’ll go back to normal. Nifty.
USB ports don’t like it when asked to source more than 100 mA. The higher power ports will acquiesce up to 500 mA, though. This circuit has a 250 mA breaker on one line and then another in parallel that can be put into play via the BOOST jumper. That is, boosted the circuit will pass 500 mA. LEDs are current sucks if you are pushing the limit, so those are optional via the LEDS jumper. Adding LEDS more than doubled the part count, but they are useful for trouble shooting.
For more info:
Selecting a PTC Resettable Fuse and TVS Zener Diode: http://forum.sparkfun.com/viewtopic.php?p=84708
Resettable Fuse PTC: http://www.sparkfun.com/products/8357