a synapse is a junction between two neurons, whereas synaptic cleft is the gap between axon terminal of a neuron and dendrites of another neuron.
a synapse is a region where communication occur between two neurons . synaptic cleft separate the plasma membrane of presynaptic and post synaptic neurons in a chemical synapse . synaptic cleft is filled with interstitial fluid.
Following diagram shows some more info about synapse :