Electricity is voltage and current. Voltage is electrical pressure, and current is the flow of charged particles. The difference between alternating current (AC) and direct current (DC) is that the electrons in an AC circuit regularly reverse their direction. In a DC circuit electrons always flow in the same direction.
According to many stories surrounding the origin of the compact disc, 74 minutes was settled on because it would comfortably contain all of Beethoven's 9th symphony on one disc. Although some stories reference a specific Sony higher-up who claimed this as his favorite piece of music, stories differ. All that's known for sure is that Sony insisted on this and won out over Phillips' original 60 minute proposal.