This page is designed as a boot camp for identifying basic logic elements on a digital IC.
CMOS logic employs complimentary transistors to achieve logic levels. Typically as one side turns on, the other side turns off, hence complementary.
BJTs typically only rely on one transistor type to perform logic. Early BJTs provided higher current output than CMOS and were more resistant to ESD. Thus, with the exception of ECL for high performance circuits, you are more likely to see CMOS based logic than BJT logic in modern circuits.