The name thyristor is derived bya combination of the capital letters from thyratron and translstor. This means that thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube. Basically thyistor is a family of power semiconductor device and SCR is an oldest member of this family that why some times we use the name thyristor instead of SCR.
SCR stand for silicon controlled rectifier. It is four layer PNPN three junctions ( J1,J2, and J3) and three terminal device. The terminals are named as anode A cathode K and gate G as shown in figure.
Figure Thyristor basic structure symbol
The terminal anode is connected to outer region the terminal cathode is connected to outer in region and the terminal gate is connected to inner P region, when a small current flows into the gate this allows a larger current to flow from the anode to the cathode. Even when the current into the gate shop the thyristor continues to allow current to flow from anode to cathode. It latches on. Thyristors are usually manufactured from silicon because of its overall properties. It also has goo thermal properties. The other reason for using silicon is that silicon technology is well established. As a result it is very cheap and easy for semiconductor manufacturers to uses.