A protein complex that consists of a homo- or heterodimer of members of a family of structurally related proteins that contain a conserved N-terminal region called the Rel homology domain (RHD). In the nucleus, NF-kappaB complexes act as transcription factors. In unstimulated cells, NF-kappaB dimers are sequestered in the cytoplasm by IkappaB monomers; signals that induce NF-kappaB activity cause degradation of IkappaB, allowing NF-kappaB dimers to translocate to the nucleus and induce gene expression.