what is the difference between anoxic and anaerobic conditions?

anoxic - means without oxygen

anaerobic - means without air (where "air" is generally used to mean oxygen)

Many times these 2 terms are used as synonymous, but in the true meaning of them, there is slight difference between those 2 terms.

In practice the difference is the method of exclusion of oxygen (air).  Anaerobic will be where the chamber is closed to outside air and no air is allowed into the chamber.  There is no air/water interface.  In anoxic, the surface of the basin is in contact with the air, but there is no active mixing of the water and the air, like when you have a basin where the water is mixed with itself, but there are no aerators.  The DO in an anaerobic system will be near 0 mg/L.  In an anoxic system, since there is a water/air interface, the DO level will be 0-1 mg/L, but near 0 mg/L.  They can be very near each other and some people will use the term anaerobic when it really is anoxic.