A sect is a small religious group that has branched off of a larger established religion. Sects have many beliefs and practices in common with the religion that they have broken off from, but are differentiated by a number of doctrinal differences. In contrast, a denomination is a large, well established religious group.
The word sect comes from the Latin secta, meaning an organized religious body or organization, from Latin, meaning a course of action or way of life.
Sociologists use the word sect to refer to a religious group with a high degree of tension with the surrounding society, but whose beliefs are (within the context of that society) largely traditional. A cult, by contrast, also has a high degree of tension with the surrounding society, but its beliefs are (within the context of that society) new and innovative. Sects, in the sociological sense, are generally traditionalist and conservative, seeking to return a religion to its (perceived) religious purity.
In European languages (other than English) the word 'sect' is used to refer to a dangerous religious sect, similar to how English-speakers popularly use the word 'cult'.