if you googled l298 arduino, you would find days worth of reading on the mutitude of times this has been documented.
here is one http://forum.arduino.cc/index.php/topic,84439.0.html
but, the shorthand...
EN-A or enable A means this pin is connected to the enable pin on the chip
5V means 5 volts. it indicates that this pin has 5 volts on it when you power the board.
it is from the on-board 5v regulator.
if you take off the 5v you disable the chip
if you put on the 5v, you enable the chip.
the speed and rotation is based on you sending a pulse stream to the input pins.
for a DC motor you use it one way, for a stepper you use it another way.
by the way, if you got one from seedstudio, you could use their documentation
http://www.seeedstudio.com/depot/datasheet/L298%20Dual%20H-Bridge%20Motor%20Driver%20datasheet.pdf
AND, they actually read the data sheet and brought out the sense resistor pins so they could be used and not burn up your motors or drain your power supply.
snort.... grunt.... going to have a second cup of coffee........ be back, in a better mood.