Cult worship is usually the worship of the evil side of a god (but can worship the good side of a god), or at least a form of worship that differences from the norm of society (if any norm exists at all), the main difference from standard worship is that cults have fewer members (sometimes because of security and secrecy reasons).
Standard worship is the worship of the god was the first of your race. Cult worship often works the same. Most cultists of Talor are actually humans . However, where the good sides of the gods often agree not to try and steal each others worshippers away, the evil side of the gods revel in stealing away loyal servants of other gods or even the good side of themselves. Thus cults are usually allot more mixed by race. Of course their are exceptions, some cults are terribly racist or elite and only take members from a certain race or class and other cults might actually worship the good side of a god and thus agree to the above rule.
Reasons to become an evil cultist
The reason to worship an evil side of a god are very varied. Evil cult worship, especially of the primary gods, usually focusses on wanton destruction, murder and the spreading of general evil, so why would you devote your life to this? One of the main reasons for people to be part of a cult are the promises of rewards given to devote cult members. Whether these rewards are eventually handed out, and if they are handed out before or after the death of the individual are highly varied, depending on the god in question, the rules of the cult, the demon(s) leading the cult and the strength of the individuals soul and devotion. Another reason is that it is usually hard to gain rewards from a good god. It is allot harder and complex to help people in need than to destroy and kill. So rewards are easier to achieve.
Cult worship is usually thought to a child of parents who are cult members. But some Cults forbid heritable membership safeguarding either the discreetness of the cult, or wanting only members who have proved themselves "worthy" in some form or another. The actual method of worship is highly varied and depends on the god worshipped.
Other reasons for joining include: Threats, misguided into thinking you are doing good deeds, sadistic, wish to be in power and control of a cult, infiltration or general insanity.
Secrecy and the influence of the evil gods
Some cults don't tell their members they are part of a cult, or only tell them after a certain time, or after they have proven themselves worthy. A very large portion of the criminal, cruel, evil or aggressive organisations, groups and families within heimr have an evil cult on top of their power structure and are thus in some way guided by demons of the evil gods.
Even if such an organisation starts out as a organisation without a cult leadership, it won't be long before a demon (sometimes in disguise) joins the organisation and eventually takes over the group from within, either helped by supernatural powers or simply because he lives longer than any other member of the group.
Reasons to become a good cultist
In tribe society no true large organized religion exists; communication between tribes is sparse and every tribe has their own little variations on the grand theme of the deity in question. Therefore the need for good cults within tribes is virtually non-existent. Other gods might have other reasons to why no central standard worshipping religion has been formed for them. Sin for instance loves the many variant ways in which he is worshipped and the discussions his worshippers have about it.
However in large cultural and civilized urban sprawls a consensus about religion usually forms after several churches fuse to form strong centralized front. This usually happens when a saint or strong divine being arises and gathers all the churches under one banner. This often leads to splinter groups and churches that, for example, don't agree with the general consensus about how a specific part of a ceremony needs to be conducted, or what is the best way to teach children the details of the faith. Sometimes these splinter groups arise right away, undermining the authority of the saint or divine being in question, claiming that they are false; maybe even demons in disguise or madmen. Sometimes these splinter groups arise after the death of a saint or departure of a divine being, claiming that "her or she would not have wanted it like this" after the general groups makes a decision without direct guidance.
These rebels (and probably their children) will thus be part of a good cult. Not even necessary on hostile terms with the larger standard worshipping group.