Short answers, pulled from the story.
Multitrack recording was conceived and developed by Ross Snyder at Ampex in 1955. Snyder built the first Sel-Sync machine, an eight-track recorder using one-inch tape, which was sold to guitarist and inventor Les Paul for $10,000.
Sel-Sync, short for Selective Synchronous recording, allows an artist to record a new track while simultaneously listening to previously recorded tracks on the same tape. This made overdubbing and layering of separate performances possible for the first time.
Early professional setups used eight tracks, as in the first Ampex machine sold to Les Paul. Studios later moved to 16-track and then 24-track recording on two-inch tape. By 1982, the rock group Toto recorded parts of Toto IV using three synchronized 24-track machines, giving access to 66 audio tracks simultaneously.
In 1963, the Beatles used twin-track machines during the recording of Please Please Me. Producer George Martin employed a bouncing technique, combining existing tracks onto a single track to free up space, until an eight-track machine became available during the recording of the band's self-titled ninth album.
Widely used multitrack recording software includes Pro Tools, Logic, Cubase, Nuendo, Ableton Live, FL Studio, Digital Performer, Cakewalk Sonar, Samplitude, and Reason. Lower-cost alternatives include REAPER and Mixcraft, while free and open-source options include Ardour and Audacity.
Multitrack recording allowed a singer to layer multiple harmony vocals over their own lead voice, a guitarist to add many harmony parts alongside their own solo, and engineers to apply different effects like reverb to individual tracks without affecting others. It also made it possible to record drums and play the tape back in reverse, creating sounds impossible to achieve with a live performance.