The way I see it, a hazer is really a fog machine and a fogger is really a smoke machine. Haze is alot less thick and is less visible, while fog is highly visible and very dense. There are probably other differences that I'm sure other people will be able to point out!
Heres when it gets tricky when it comes to fog and haze. Basically what is going on with haze machines is that once it passes through the heat exchanger in a haze machine, thats it, it is then passed through a fan, keeping in mind that heat rises and when the fan hits it it disperses it creates an even haze outoput in area respectively of the output of the machine output and fan speed. On the other hand once the fluid is passed through fog machine heat exchanger it is actaully cooled down some, this is where you tend to see the fog acutally hug the ground better than haze. You could use a fog machine and a fan to disperse the fog to create a haze which works fairly well in large outdoor areas where large clouds of fog might be hard to notice, but on an indoor smaller stage you will def. be able to see spots where the fog tends to gather more and not disapate as quickly. Also be sure to watch which fluids you use, they make fluids that do a lot of different things such as hang times, and density, etc...so I would recommend using a haze machine for an indoor play or event because of the dispersion pattern.
Let me know if you need anything else.
Actually.... Fog does not get cooled down. There is the Jem Glaciator, but that's about it. Fog comes out hot, and rises. I Know my Fog and Haze machines really well (I've torned them apart a few times). I know i use fog for dense effects, but i wonder why haze machines exist because i could just use a fog machine with fan.
For the record I own a Rosco 1600 fogger and I borrow a LaMaitre hazer sometimes. I always use the fogger unless they specifically call for a hazer.