BOP rams are placed inside of the blowout preventers and are used in well control situations. Rams cut off flow from the well during a kick by sealing the annulus space.
Blind rams are used when there is no pipe present in the BOP at the time of a well control situation and close the annulus off completely.
Shear rams are used to cut through drill pipe that is in the BOP and are used as a last resort in a well control situation.
There are other types of rams that are used such as dual rams which can close on two different strings in the BOP and Variable Bore Rams (VBRs) which close on different sizes of drill pipe.
BOP rams are specific to the type of BOP on the rig.