Wye is where the 3 phases are referenced to a central grounded point, looks like a "Y". That is your normal 208.
Open delta, delta "Vee" etc is a delta that skips one of the transformers. Imagine a triangle where you erased one of the sides. That is typically 240v and they centertap one of the secondaries for the ground.
The voltages will be 120/120/208 to ground and 240 between phases.
Open delta is a cheap way to serve a place that needs 3 phase and also 120v since it only takes 2 transformers.
Balancing the phases can be tricky but if done well it works.
