Neuromodulators modulate regions or circuits of the brain. That is, they affect a group of neurons, causing a modulation of that group.
In contrast, neurotransmitters affect one other neuron. Their action can be described as a neuron-neuron exchange of information, rather than an action on specific postsynaptic neurons. Therefore neuromodulators differ from neurotransmitters, from the point of view of their defining actions. But many neuromodulators do also act as neurotransmitters.
See also: neurotransmitters for a list of NMs/NTs.