I've designed the board maybe I can answer
btw in order to understand what the components do, you need to refer to the schematic diagramhttp://www.potemkin.org/uploads/Pid/arduino_usb.jpg
a. power led, it lights up when the board is powered. Normally it's green. Some boards have a red led because of a mistake done by the assembly company.
b. ISP connector. ISP means "In System Programming" this connector allows programming the microcontroller directly. This is used to program the bootloader on the processor before the board can be used.
c. reset button
e. protection diode. prevents applying the power to the board with the wrong polarity (swapping + with - can damage the processor)
f. capacitor. sometimes the circuit you attach to the board might have a sudden power surge that will drop the voltage and below a certain value the processor might reset. this capacitor kicks in a provides that extra bit of power to compensate for the fluctuactions.
g. RX TX leds. these light up when the board receives or sends data from the USB. it's useful to debug issues with USB communication.
h. 16MHz quartz. this provides the clock signal to the processor.
i. 5v out. this provides a source for regulated 5V supply. where this come from is dependent on SV1 (letter o)
j. and k ground connection (the - on a battery). this is connected to the ground path on the board. why 2 ? because 2 is better than 1 (infact we have 3 since letter w is also ground)
l. 9V. this is the voltage that has been applied to the dc power plug (letter r) normally it's around 9v but it depends on the power supply you're using.
m. analogue inputs. see http://arduino.berlios.de/index.php/Tutorial/JoyStick
for an example of use.
n. USB connector. It's an USB 2.0 (not high speed. 12 MBit max) connector that brings data into the board.
o. power selector jumper. this jumper decides where the power comes from. between pin 2 and 3 it takes the power from the USB bus while between pin 1 and 2 takes power from and external power supply.
p. ceramic resonator. this provides the 12 MHz clock to the usb interface chip.
q. power regulator. when the board is powered from the external power supply this makes sure that the voltage applied to the processor is always 5v
r. external power plug. this is where you plug an external power supply. the size is 2.1mm with the plus at the centre pin. power applied to this plug should go from 9 to 15 volts.
s. resistors. these are required to interface to the USB bus.
t. usb to serial interface chip. this chip allows the microcontroller to connect to the USB bus and use it to talk to the computer.
u. capacitors. these are used by the usb interface chip (letter t)
v. aref. this is a tough one... this pin is used to tweak the way the analogue pin translate voltage into numbers. it's used only by advanced user.
x. digital pin number 13. the only difference from the others is that this one has an 1k resistor attached to it so that we can plug an LED directly from 13 to ground without burning it.
y. these are special pins controlled by the USB bus. they will be used in the future to automatically reset the board before exporting. this will save users from having to press the reset button all the time.
z. resistors. the one you are pointing at is used to limit the current that goes into the RX TX led.
hope this helps...
You'll learn more from the schematic when you know how to read it