All synapses are junctions between neurones and cells. Neurotransmitters are released from the presynaptic neurone into the synaptic cleft, and then they bind to the postsynaptic membrane and stimulate action potentials in postsynaptic neurone.
Cholinergic synapses are simply synapses that use Acetylcholine (ACh) as their neurotransmitter. They are an important kind of synapse because they are so widespread in the body, passing on signals to muscle cells in all neuromuscular junctions.