An SCR is a diode which when idle is practicallly open circuit both ways. It has a trigger line, which when triggered (with a low current), causes the diode to conduct in the forward direction, as long as current continues to flow. When current ceases to flow (or flows below a point), the SCR falls "open", waiting for the trigger to happen again.
A triac is basically two SCRs back to back, with the trigger lines tied together. Triacs can pass AC power (because the current flows in both directions), so are used for AC lamp dimmers and motor controls.
An SCR is only one way, so would only pass one half of an AC waveform. Because an AC waveform goes to zero, the SCR or triacl would fall "open" and need triggered each half cycle.
A DIAC is a two way diode, which passes current in either direction, but only above a certain voltage point.