The word “cult”, in its original sense, refers to a collection of practices and rituals associated with a religion or with a particular aspect of a religion. This is how the word is primarily used in academic discourse. Thus we can speak of the cult of a particular saint in reference to the devotions associated with honoring that saint in Catholic Christianity, or the cult of worship of Vishnu in Hinduism, for example.
However, in more popular usage, the word “cult” has come to refer to exclusive or separatist religious groups, especially new groups, whose beliefs and practices fall outside the perceived mainstream. The word is often used pejoratively, to label a group’s expressions as threatening or deviant. Scholars of religion avoid this sense and instead refer to such groups as “new religious movements.”
The Catholic Church has a nuanced understanding of the beliefs and expressions of other religious groups. We recognize that other religions traditions can be a source of life and light for their adherents. Rather than labeling any group as a cult, we teach that whatever is good and true in other religions is in fact a preparation for the gospel. At the same time, we denounce any elements that run contrary to the goodness and truth of the gospel – including some destructive elements of new religious movements that may be popularly known as cults. The Dogmatic Constitution on the Church, from Vatican II, offers an excellent summary of the Catholic perspective on how followers of other religions are related to the Catholic Church and the saving promises of Christ in #15-16. You can find the text of the Dogmatic Constitution on the Church at http://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19641121_lumen-gentium_en.html.