There are two extremes:

 - Play every note live (as acoustic musicians do)
 - Press 'play' on a recording

Most electronic musicians choose their own method, that's somewhere on the continuum between those two extremes.

Techniques include:

 - Playing parts live
 - Looping pre-prepared sequences
 - Manually triggering transitions from one loop to another
 - Manually manipulating parameters on synthesisers and effects

Some performers simply manipulate a laptop. To me this has two disadvantages -- it's too limiting an interface for the performer, and it's not visually interesting enough for the audience. To me it's more satisfactory when there are physical knobs and buttons and keys, and that can be achieved by having real outboard synths and effects units with their own knobs -- or by having MIDI controllers hooked up to the computer.

A couple of examples:

The Vancouver band *Holy Fuck* are a four-piece consisting of a drummer, a bassist, and two musicians who work the electronics. The drummer plays real acoustic drums. The bassist plays a real electric bass. They supply a genuine live feel to the music. The other two work at trestle tables filled with circuit-bent "toy" keyboards, guitar effects and so on. They work with "loops" in the sense that they use heavily distorted preset patterns from the keyboards, but they also play those toy keyboards, or simply coax drones out of them. They also sing or hum into microphones, again heavily effected. Their time is spent starting and stopping patterns, playing keyboards, and adjusting effects.

They make a tremendous sound, and it's unusually entertaining to watch, for an "electronic" act, since the rhythm section is live, and all four use eye contact and gestures to synchronise each other.

The well known techno act *Orbital* consists of two men, working with sequenced synths, drum machines and effects. I'm going to talk about their "classic period" live act, because that's what I've observed most closely. I know since then they've moved on to using Cubase, although that may not make much difference.

Orbital worked with hardware MIDI sequencers, driving hardware synths. I *think* they had a sequencer each, synchronised, so they could each be triggering loops or transitions. A performance involved both of them constantly hitting buttons or twiddling knobs, to prepare the next pattern, trigger it, tweak the sound of a synth or an effect, and so on.