The word "Jhana" means "Absorbtion". It's a state of mind where Samadhi (Concentration) reaches some frutation. There are 4 states of Jhana (the Buddha was said to have attained all 4 on the same day just before Enlightenment).
This is the Buddha's description of the First Jhana. It's not verbatim, but is as close to verbatim as my memory allows:
"The meditator comes upon this state of mind. The Five Hinderances (Ill-will, Laziness, Restlessness, Doubt and Sensual Desire) disappears without remainder (temporarily). The meditator feels a great peace and happiness filling his entire mind and body. However, the mind is still filled with wandering thoughts. This is the First Jhana....."
The 2nd Jhana is the same as the 1st, except that the mind is perfectly concentrated. In the 3rd Jhana, there is no more feeling of "joy", but a "Rapture" occurs in which there is only peace, no "joy". I'm unqalified to discuss the 4th Jhana, but one can find its description in many sutras in the Pali canon.
Jhanas are an important part of a practitioner's path. The Buddha would even go so far to say that, "No Jhana, no Nirvana". However, Jhanas are just temporary states. The Buddha and many meditation teachers ever since had warned of the dangers of Jhanas. One Teacher even described it as "the most dangerous of samadhi states". The reason is that Jhana is impermanent. It, and all its good things, arises only during those meditations. The 3 dangers of Jhana are:
1. Meditators get attached to it and neglect to continue working towards
the true path, thinking that the goal has been reached.
2. When Jhana states do not arise during some meditations, the meditator thinks, "I'm not getting that wonderful feeling, I must be doing it wrong, I must be very stupid ......". This causes doubt and guilt.
3. Jhana states do not translate very well to "normal" life. One can attain Jhana today and still get involved in a fist-fight the next day. Only thru further cultivation of the mind can the states of peace be "permanent".
Jhanas are powerful experiences. In no other experience can an unenlightened person experience the temporary destruction of the 5 Hinderances and the peace and happiness that comes with it. For many, it may even change their lives. However, if one does not realize the danger of Jhanas, they may cause much more harm than good.