Electricity is generated and distributed as 3 phase.
There are 2 ways to connect a load to a 3 phase supply:
"Delta" or "wye"
Delta connections are between the phase line wires (no neutral, no earth)
wye connects between a neutral and each line wire. The neutral can be connected to earth at the transformer*.
The supply transformer's 240V output coil is centre tapped to give a 120(L1)-Neutral-120(L2) supply which is known as "3-wire single-phase".
A polyphase supply brings in a supply from more than 1 transformer coil. This can be 2 or 3 phases. The voltage between the Line wires of the different phases will be be 208V (120 x 1.732#). Such a supply may be labelled 120Y/208. There could be any combination of 120 and 208 and 240 Volts available. Higher power devices are available in 208 and 240 V versions in the USA
For more heavyweight (industrial) users the supply transformer output coils can be providing 240, 480, 600 volts (120, 208 and 277 volt supplies can be obtained from these).
Large motors will be 3 phase and are usually delta connected.
There is no NEC requirement for colour coding*# so it's not easy to know what sort of supply is being dealt with. Black is often used for 120V hot, and red for 208V, but this is not universal. (No wonder even electricians can get confused)
Earth and Neutral
Most devices will be earthed. This is not the same as connecting neutral to earth.
* How and where the neutral is connected to earth varies according to differing local codes and practices.
# Phases in a 3 phase system are seperated by 360/3 = 120 degrees. If you plot graphs of 3 sine waves of ampltitude 1, spaced at 120 degrees, the ampliutude between them is 1.732. (√3)
*# NEC requires: Ground = Green or Green/Yellow; Neutral=White; High Leg "must be durably marked"