What is a differential probe?

A "differential" probe is an active probe that has two inputs, one positive, one negative, and a separate ground lead; it drives a single-ended 50-Ω cable to transmit its output to the oscilloscope channel. The output signal is proportional to the difference between the voltages appearing at the two inputs. Differential probes reference each other, rather than ground, and observe small signals in the presence of large DC offsets, or other common signal patterns such as power transmission line noise.

You would typically choose a single-ended active probe to measure single-ended signals (voltage relative to ground) and a differential active probe to measure differential signals (positive voltage versus negative voltage).

One thing to keep in mind, however, is that the effective grounding between the signal connections in a differential probe is better than the large ground plane in a single-ended probe. This grounding effectively connects the probe ground lead to the device under test (DUT) with very low impedance. Therefore, a differential probe will make better measurements on single-ended signals than a single-ended probe.