FX Loop:
This allows effect pedals to be patched into the amp between the preamp and the power amp, instead of connecting them between the guitar and the preamp input. The FX loop is a good place for time-based effects like reverb, delay, echo, and flanging. Distortion and wha-wah still sound best between the guitar and the amp.

MIDI:
Musical Instrument Digital Interface circuits allow for computer-controlled switching of effects patches, equalizer settings, channel selection, samples, or anything else that has a MIDI control feature. Often used to switch pre-sets on complicated multi-FX units or modelling amplifiers.