OPC is the most commonly used type of cement in the world. OPC is the basic form of cement produced by inter-grinding cement clinker with 3-5% gypsum - which is introduced to enhance the setting time of the cement to a workable 30 minutes or so.
The grade number indicates the minimum compressive strength that the cement is required to attain at the end of 28 days eg, the minimum compressive strength of 53 Grade OPC attained on the 28th day shall not be less than 53 MPa or 530 kg/sqcm.
It may be noted that OPC 43 and OPC 53 grades do not differ in chemical content. The only difference is that the higher grade cements are ground much finer during the final grinding process, creating a product that is much stronger and more durable than the less finely ground cement.