Japa is the continuous mental repetition of a mantra. Mantra is a powerful tool for focusing the mind.
Japa can be practiced in any place or situation. This will help a great deal with those times when the mind has a tendency to focus on the outside world. When the recitation of Japa becomes spontaneous, this will bring happiness.
Chanting Japa from the heart and not as a mere repetition will lead to states of ecstasy.
A mala can be used when chanting Japa. A mala is a necklace with beads that help with the count of mantra.
The difference between Japa and mediation is that Japa will bring your mind to a one-pointed state while meditation is pure silence of the mind.