To start with- lets build a foundation to make what follows clear:
Moses received the Torah on Mt. Sinai from God. The Torah consists of two parts
1) The written part, the Chamishe Sifrei Torah (the five books of the Torah- Five books of Moses)
2) The oral law, Mishnah, which was written down after the destruction of the second temple so it would not be forgotten or altered through inaccurate transmission.
The rest of the Tanach (Torah, Nevi'im(prophets) and Ketuvim(Writings)) came later. The origins of the Kabalah are in there- the description of the heavenly chariot is considered one of the major mystical portions- and from which a lot of Kabalah is learnt, and some claim that the entire book of Job is one of Sod (literally secret and referring to Kabalistic meanings). Later, the Serfer HaYesod was written- which was then complemented with a far more complete work, the Zohar. But these books are not complete- somethings are not written down. These form the basis of what is known as Lurianic Kabalah, named after Rabbi Isaac Luria (Ha'arizal) who is considered one of, if not the, greatest Kaballist in history. A competing school of Kaballah is based around the teachings of the Ba'al Shem Tov, the originator of the chassidic movement, and summarised in the Tanya, written by Rav Schneur Zalman of Liadi- the founder of the Chabad Lubavitch movement (Chabad literaly stands for "Chochmah, Binah and Da'at- the three main sefirot in the tree of life as taught by the Ba'al shem tov)
When jews study the Torah- it is looked at in various ways- and in each way, a word, or even a letter can mean something different or teach something different.
1) Pshat- this is the plain, easily iunderstood meaning (yeah right- sometimes even this is difficult!)
2) Halachic - the legal imterpretation- so taking the legal definition of a word rather than its straight meaning. This is really just a subset of pshat since they are both plain meanings though what is learnt may differ.
3) Remesh - the alluded to meanings. This is where you get the alluded to meanings and the oral law meanings of the passages. Some of these are aggadot- more like morality stories or fables- though always meant to teach, others are halachic and give guidance on the laws.
4) Sod- secret. This level of study is the most difficult and is not common. studying at this level is usually only done by a student and teacher in an one on one session and is not taught in large groups or classes. the reason for this is that the teacher has to make sure that the student fully understands what is being taught, ot the student may be led astray. A story in the Talmud, masechta Chagigah, is told of Rabbi Akivah, one of the greatest sages who, using kaballah from the lessons derived from the vision of the merkava (divine chariot), ascended to view the world to come, with four students- each a great sage in their own right. One student who was poure, did not guard himself and died from the view, a second went mad, a third died and the fourth became an apostate and started his own religion, dualistic in nature. The Talmud brings this story to teach 1) that this should never have been done in a group and 2) as a general warning that studying Kaballah is not for everyone.
So let us now look at Sod- the area in which Kaballah is found.
a) The earliest written work of Kaballah is generally stated as being the Book of Job. The Rabbis do not view this book literally but rather as an allegory in which many Kaballistic insights are taught.
b) The earliest oral source of Kaballah is stated as being the Sefer Yetzira, which tradition states was authored by Abraham and passed down orally until it waas written down around 200CE since it was in danger of being corrupted or forgotten.
c) The vision of the divine Chariot
d) The Zohar - Tradition states that this was a compilation of lessons that Rebbe Shimon Bar Yochai collated and organised while he was hiding from the Romans around 200CE. Because it is in the realm of Sod- people were always reluctant to write down- but it was finally written down around the 1600s.
Who studies it?
Basically the majority of Orthodox Jews accept Kaballah as worthwhile studying, though there is a rule that we never alter the halachah (Jewish law) because of what is found in the Kaballah. The Torah, written and oral, takes primacy.
However, Judaism is focussed on action- not belief, in doing, not in studying for the sake of studying. As such, the focus has always been on learning the Torah first- together with all the laws and how to perform them properly, before studying anything to do with the Kaballah. Thus for a long time no Kaballah was written down- and it was only directly taught by teacher to student in an one on one fashion. even today- the written works of the Kaballah do not contain everything- their are major elements that are only taught by teacher to student. As such- finding a teacher is paramount- and it is not easy to do, Generally, a teacher will only take a student who is married, has children, has studied all of shas (the complete Jewish law) and is living an observant Jewish life style. Studying Kaballah outside of Judaism is a bizarre concept- much of Kaballah is direct commentary on the Torah and Tanach (such commentary is "remesh" and refers to the hidden meanings in the verses as opposed to the p'shat (direct) meaning. Other major sources of learning remesh are the Midrash Rabbah, Sifrei and Sifri. The commentaries of the Ramban and Ba'al Haturim are largely based on remesh as compared to Rashi who exlictly went out to only give the p'shat meaning of the Tanach. )
So Kaballah can be genuine- but most of the time what is taugh is far from genuine Kabalah. The Kabalah centre of Philip Berg is a scam and cult. (I don't refer to him as Rabbi since the place he claims to have gotten ordination from states he never got ordination! Considering what he is doing, I believe them- not him!) Red strings, holy water, reciting verses without understanding them are all nonsense. The other people teaching Kaballah out there are just as fake (though not all of them are as destructive as Berg's cult). Madonna might be happy there- good for her, but what she is studying is just something made up by someone with a very superficial understanding of but a few issues- and then elaborated on in a huge labyrinth of fakery to part people from their money!