LEDs are Light emitting Diodes, LEDs are rarely used as diodes but used more often as light emitting indicator. LEDs lits when connected in forward bias and switches off emitting light when connected in reversed biased. Other characteristics of LEDs are very much similar to any general purpose diode. As shown in the image below, they come in many colors. Some are even bi and multi colored.

There is a simple way to identify anode and cathode of a new diode without using multimeter. If you observe the two pins closely as shown below you will find that one pine is longer than other, the longer pin is generally anode and shorter one is cathode. This rule applies only to new diodes because for the used diodes taken from old circuit, one cannot say any thing just by looking a pin lengths.

10 mA of current is enough for normal brightness but always remember to connect LED with proper register to limit the current value around 10 mA as shown in the circuit below. Resistor value can be calculated using formula: R = (V -V_led) / I

Where V is battery voltage, V_led and I(current) should be referred from LED data sheet or can be taken as 0.7V and 10 ms respectively for general calculation.