Sundar,
Subject GRE tests are conducted by ETS and are meant to, as per ETS, "to help graduate school admission committees and fellowship sponsors assess the qualifications of applicants in specific fields of study”.
Subject GRE tests are subject specific, timed, multiple choice question tests and are conducted in Physics, Chemistry, Maths, Biology, Bio chemistry, Computer Science, Psychology and English Literature.
If you are planning to seek admission in these subjects, it is strongly recommended that you appear in the respective subject gre test.
Cheers