Sense amplifier
Beginner
A sensitive detector that notices a tiny change on a wire and turns it into a clear 1 or 0.
Novice
A circuit at the end of each bit line (or bit-line pair) that detects a small voltage difference and amplifies it to a full logic level. It lets the memory finish a read without waiting for the cell to swing the long bit line all the way.
Expert
Usually a clocked cross-coupled latch: fired once the bit-line split exceeds its offset plus noise, it regenerates to full rail. Its input offset (from transistor mismatch) sets the minimum swing, so it ties read speed to variation. In DRAM it also writes the value back into the cell.