Below is a list of five traits that many cults tend to have:
(It should be noted that not all cults contain all five of these traits, but many do, and most other cults contain at least four out of five of these traits)
Perhaps one of the most well known traits of a cult is the isolation of it's members. Isolation can include things such as encouraging, or even enforcing it's members to lessen, or have no contact at all with people who criticize the cult, including friends and even family.
Some of this is even more extreme, in which cults not only encourage isolation from friends and family, but from society itself. This can include anything from just having only contact with other members of the cult, and having as little contact with anyone else as much as possible, to living away from society in compounds.
Another form of isolation that cults sometimes engage in is to make it's members act in such an obscene, hostile, annoying, or even forceful manner, that no one wants to be near them.
Cult leaders often times encourage their members to give up large sums of money, usually through either fraud programs that makes a person believe they will advance in the cult, or as a form of donation to help advance the cult itself, or in some cases, both.
One of the other ways cult's get money from their members, and even non-members who may just have an interest in the cult's teachings, is to sell overpriced products, such as books that promote the cult's teachings, or overpriced services that really do nothing. Sometimes it is not even cult members and people who are interested in the cult that buy these things, but sometimes it's just ordinary people who get scammed into buying stuff from these cults.
Other ways cult leaders control people through finical means is to "encourage", or more often times try to force it's members to do work for the cult, usually for either little, or no pay. The difference between this and volunteering is that with volunteering is that you can leave whenever you want. With a cult, this might not be an actual option, and they might try to prevent you from leaving, and can include anything from being threatened, to being physically kept from leaving.
3. Extreme Negative Reaction to Criticism
Many cult members react very negatively toward criticism. Most often times this reaction to criticism comes in the form of insults, threats, or long, sometimes copied and pasted posts on internet message boards and comment sections, usually as an attempt to silence a critic through either fear, or posts of attrition. Also this is sometimes done as a way to spread their cult's propaganda.
Other types of reactions include much more hostile, and many times, illegal forms of negative reactions, such as harassment, slander, theft, vandalism, blackmail, and filling frivolous lawsuits. Sometimes this can even include violent actions, such as assault, and even in some rare cases, murder.
2. Engaging in Dangerous, Destructive, or Illegal Behaviors
One of the most common things cult leaders often encourage their members to do is to engage in actions and behaviors that can be considered dangerous, destructive, and even illegal. Many of these behaviors include greatly encouraging, or intimidating members into not engaging in certain medical practices that are often times life saving, or to engage in certain medical practices that are unnecessary, or to engage in alternative types medical practices that often times do nothing, or does more harm then good. Often times these programs that offer such medical practices are run by the cult itself, or members of the cult.
Another type of dangerous behavior often times encouraged by cults, is for members to tell their secrets to other member. Most of the time this is claimed as a way to "cleanse" ones self. What this is actually used for, most of the time, is to gain information about a member that can be later used to blackmail them, if need be.
Other types of destructive and illegal actions and behaviors also sometimes include forcibly isolating members, and holding them against their will, usually when they're showing signs of mental illness, or just wanting leave the cult. Still, some other types of illegal actions can include cult sanctioned abuse, both physical and sexual. In the most extreme cases, cult leaders sometimes talk about having their members commit suicide, or commit murder.
Most cults are very secretive, and many cult members are "encouraged", or forced, not to reveal certain teaching, or practices, within the cult to outsiders, or they don't want certain illegal activities to catch the attention of authorities. Threats of lawsuits, being kicked out of the cult, and violence, are often very common.
Many times it's not just the teachings and practices cult leaders don't want revealed, it's the actions of members within the cult itself, and usually it's illegal actions they want to cover up.
The reason why most cults are so secretive is most likely because if any negative things get revealed, then this tends brings a lot of negative attention to a cult, and sometimes it can bring down a cult and destroy it, either by authorities, or by members leaving.