Delta wound has the applied voltage across one winding where as wye wound the applied voltage is across 2 windings at any given time.

In industrial applications some motors can be configured to be wye or delta often combined with controllers to switch the configuration whilst the motor is running. IE they use wye to start the motor with reduced starting current then switch on the fly to delta wound for maximum power.

A little rusty (I haven't done this for 8 years) but the number of turns changes by approximately square_root(3) or 1.73 for the same motor delta or wye wound.

Most of which is irrelevant for RC cars. From what I have seen with an equivalent power motor, delta wound motors are a little smoother which can be important in MOD.