Pretty straightforward: what is the difference between forward flight, straight flight, level flight, and cruise flight in helicopters?
Forward flight is how the helicopter flies in most situations. It flies similar to an airplane in this situation, with pitching up and down to increase and decrease airspeed. This is controlled by use of forward and aft cyclic.
Straight flight is where the tail rotor is set so that it opposes the turning tendency of the main rotor. This essentially prevents the helicopter from yawing while in flight. This is controlled by using the anti-torque pedals.
Level flight is simply when the helicopter is flying at a constant altitude. This does not mean, however that its nose is pointed ahead, or that it is flying forward, merely that it is not gaining or losing altitude.
I'm not sure exactly what cruise flight is. When a helicopter is "cruising", it most likely is maintaining altitude and flying straight ahead, but I don't think there is a separate term for it.
Forward flight in a helicopter means that the the horizontal component of lift is forward of vertical, and no turning component is present. Contrast to turning flight.
Straight flight is forward flight, with the yaw string centered. Contrast to a slip or a skid, or sidways or backwards flight.
Level flight means that the helicopter maintains altitude.
Cruise flight in a helicopter is the flight regime between departure and approach, at a power setting optimized for traveling between locations.